Seal implicitly used classes (#309)

Apparently the `[UsedImplicitly]` annotation suppresses the "Class has
no inheritors and can be marked sealed" warning. Cool to know.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

TeamOctolings.Octobot/Responders/GuildLoadedResponder.cs

@@ -19,7 +19,7 @@ namespace TeamOctolings.Octobot.Responders;
 ///     has <see cref="GuildSettings.ReceiveStartupMessages" /> enabled
 /// </summary>
 [UsedImplicitly]
-public class GuildLoadedResponder : IResponder<IGuildCreate>
+public sealed class GuildLoadedResponder : IResponder<IGuildCreate>
 {
     private readonly IDiscordRestChannelAPI _channelApi;
     private readonly GuildDataService _guildData;

TeamOctolings.Octobot/Responders/GuildMemberJoinedResponder.cs

@@ -18,7 +18,7 @@ namespace TeamOctolings.Octobot.Responders;
 /// </summary>
 /// <seealso cref="GuildSettings.WelcomeMessage" />
 [UsedImplicitly]
-public class GuildMemberJoinedResponder : IResponder<IGuildMemberAdd>
+public sealed class GuildMemberJoinedResponder : IResponder<IGuildMemberAdd>
 {
     private readonly IDiscordRestChannelAPI _channelApi;
     private readonly IDiscordRestGuildAPI _guildApi;

TeamOctolings.Octobot/Responders/GuildMemberLeftResponder.cs

@@ -15,7 +15,7 @@ namespace TeamOctolings.Octobot.Responders;
 /// </summary>
 /// <seealso cref="GuildSettings.LeaveMessage" />
 [UsedImplicitly]
-public class GuildMemberLeftResponder : IResponder<IGuildMemberRemove>
+public sealed class GuildMemberLeftResponder : IResponder<IGuildMemberRemove>
 {
     private readonly IDiscordRestChannelAPI _channelApi;
     private readonly IDiscordRestGuildAPI _guildApi;

TeamOctolings.Octobot/Responders/GuildUnloadedResponder.cs

@@ -12,7 +12,7 @@ namespace TeamOctolings.Octobot.Responders;
 ///     Handles removing guild ID from <see cref="GuildData" /> if the guild becomes unavailable.
 /// </summary>
 [UsedImplicitly]
-public class GuildUnloadedResponder : IResponder<IGuildDelete>
+public sealed class GuildUnloadedResponder : IResponder<IGuildDelete>
 {
     private readonly GuildDataService _guildData;
     private readonly ILogger<GuildUnloadedResponder> _logger;

TeamOctolings.Octobot/Responders/MessageDeletedResponder.cs

@@ -18,7 +18,7 @@ namespace TeamOctolings.Octobot.Responders;
 ///     to a guild's <see cref="GuildSettings.PrivateFeedbackChannel" /> if one is set.
 /// </summary>
 [UsedImplicitly]
-public class MessageDeletedResponder : IResponder<IMessageDelete>
+public sealed class MessageDeletedResponder : IResponder<IMessageDelete>
 {
     private readonly IDiscordRestAuditLogAPI _auditLogApi;
     private readonly IDiscordRestChannelAPI _channelApi;

TeamOctolings.Octobot/Responders/MessageEditedResponder.cs

@@ -20,7 +20,7 @@ namespace TeamOctolings.Octobot.Responders;
 ///     to a guild's <see cref="GuildSettings.PrivateFeedbackChannel" /> if one is set.
 /// </summary>
 [UsedImplicitly]
-public class MessageEditedResponder : IResponder<IMessageUpdate>
+public sealed class MessageEditedResponder : IResponder<IMessageUpdate>
 {
     private readonly CacheService _cacheService;
     private readonly IDiscordRestChannelAPI _channelApi;

TeamOctolings.Octobot/Responders/MessageReceivedResponder.cs

@@ -11,7 +11,7 @@ namespace TeamOctolings.Octobot.Responders;
 ///     Handles sending replies to easter egg messages.
 /// </summary>
 [UsedImplicitly]
-public class MessageCreateResponder : IResponder<IMessageCreate>
+public sealed class MessageCreateResponder : IResponder<IMessageCreate>
 {
     private readonly IDiscordRestChannelAPI _channelApi;