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/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>