Split extension methods into separate classes (#161)

This PR splits the extension methods contained in `Extensions.cs` into
separate classes in the `Octobot.Extensions` namespace. This was done
for multiple reasons:
1) The `Extensions.cs` violates SRP (Single Responsibility Principle) -
it takes upon itself every extension method for many types
2) Having a separate class for each extended type is a standard practice
- take a look at
[Remora.Discord](https://github.com/Remora/Remora.Discord/tree/main/Backend/Remora.Discord.Rest/Extensions)
or [osu!](https://github.com/ppy/osu/tree/master/osu.Game/Extensions)
3) Having all extension methods in one file makes it hard to find the
method you want

src/Commands/AboutCommandGroup.cs

@@ -2,6 +2,7 @@
 using System.Text;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;

src/Commands/BanCommandGroup.cs

@@ -2,6 +2,7 @@ using System.ComponentModel;
 using System.Text;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Octobot.Services.Update;
 using Remora.Commands.Attributes;
@@ -53,7 +54,7 @@ public class BanCommandGroup : CommandGroup
     /// <param name="target">The user to ban.</param>
     /// <param name="duration">The duration for this ban. The user will be automatically unbanned after this duration.</param>
     /// <param name="reason">
-    ///     The reason for this ban. Must be encoded with <see cref="Extensions.EncodeHeader" /> when passed to
+    ///     The reason for this ban. Must be encoded with <see cref="StringExtensions.EncodeHeader" /> when passed to
     ///     <see cref="IDiscordRestGuildAPI.CreateGuildBanAsync" />.
     /// </param>
     /// <returns>
@@ -196,7 +197,7 @@ public class BanCommandGroup : CommandGroup
     /// </summary>
     /// <param name="target">The user to unban.</param>
     /// <param name="reason">
-    ///     The reason for this unban. Must be encoded with <see cref="Extensions.EncodeHeader" /> when passed to
+    ///     The reason for this unban. Must be encoded with <see cref="StringExtensions.EncodeHeader" /> when passed to
     ///     <see cref="IDiscordRestGuildAPI.RemoveGuildBanAsync" />.
     /// </param>
     /// <returns>

src/Commands/ClearCommandGroup.cs

@@ -2,6 +2,7 @@ using System.ComponentModel;
 using System.Text;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;

src/Commands/Events/ErrorLoggingPostExecutionEvent.cs

@@ -1,5 +1,6 @@
 using JetBrains.Annotations;
 using Microsoft.Extensions.Logging;
+using Octobot.Extensions;
 using Remora.Discord.Commands.Contexts;
 using Remora.Discord.Commands.Services;
 using Remora.Results;

src/Commands/Events/LoggingPreparationErrorEvent.cs

@@ -1,5 +1,6 @@
 using JetBrains.Annotations;
 using Microsoft.Extensions.Logging;
+using Octobot.Extensions;
 using Remora.Discord.Commands.Contexts;
 using Remora.Discord.Commands.Services;
 using Remora.Results;

src/Commands/KickCommandGroup.cs

@@ -1,6 +1,7 @@
 using System.ComponentModel;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;
@@ -49,7 +50,7 @@ public class KickCommandGroup : CommandGroup
     /// </summary>
     /// <param name="target">The member to kick.</param>
     /// <param name="reason">
-    ///     The reason for this kick. Must be encoded with <see cref="Extensions.EncodeHeader" /> when passed to
+    ///     The reason for this kick. Must be encoded with <see cref="StringExtensions.EncodeHeader" /> when passed to
     ///     <see cref="IDiscordRestGuildAPI.RemoveGuildMemberAsync" />.
     /// </param>
     /// <returns>

src/Commands/MuteCommandGroup.cs

@@ -2,6 +2,7 @@ using System.ComponentModel;
 using System.Text;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Octobot.Services.Update;
 using Remora.Commands.Attributes;
@@ -50,7 +51,7 @@ public class MuteCommandGroup : CommandGroup
     /// <param name="target">The member to mute.</param>
     /// <param name="duration">The duration for this mute. The member will be automatically unmuted after this duration.</param>
     /// <param name="reason">
-    ///     The reason for this mute. Must be encoded with <see cref="Extensions.EncodeHeader" /> when passed to
+    ///     The reason for this mute. Must be encoded with <see cref="StringExtensions.EncodeHeader" /> when passed to
     ///     <see cref="IDiscordRestGuildAPI.ModifyGuildMemberAsync" />.
     /// </param>
     /// <returns>
@@ -213,7 +214,7 @@ public class MuteCommandGroup : CommandGroup
     /// </summary>
     /// <param name="target">The member to unmute.</param>
     /// <param name="reason">
-    ///     The reason for this unmute. Must be encoded with <see cref="Extensions.EncodeHeader" /> when passed to
+    ///     The reason for this unmute. Must be encoded with <see cref="StringExtensions.EncodeHeader" /> when passed to
     ///     <see cref="IDiscordRestGuildAPI.ModifyGuildMemberAsync" />.
     /// </param>
     /// <returns>

src/Commands/PingCommandGroup.cs

@@ -1,6 +1,7 @@
 using System.ComponentModel;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;

src/Commands/RemindCommandGroup.cs

@@ -2,6 +2,7 @@ using System.ComponentModel;
 using System.Text;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;

src/Commands/SettingsCommandGroup.cs

@@ -4,6 +4,7 @@ using System.Text.Json.Nodes;
 using JetBrains.Annotations;
 using Octobot.Data;
 using Octobot.Data.Options;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;

src/Commands/ToolsCommandGroup.cs

@@ -3,6 +3,7 @@ using System.Drawing;
 using System.Text;
 using JetBrains.Annotations;
 using Octobot.Data;
+using Octobot.Extensions;
 using Octobot.Services;
 using Remora.Commands.Attributes;
 using Remora.Commands.Groups;