Refactor proxy handling code

- Move reaction handlers to the ReactionAdded event instead of
  ProxyService
- Split tag matching off into ProxyTagParser
- Split autoproxy matching off into Autoproxier
- General cleanup and simplification

PluralKit.Core/Services/IDataStore.cs

@@ -281,7 +281,7 @@ namespace PluralKit.Core {
         /// Deletes messages from the data store in bulk.
         /// </summary>
         /// <param name="postedMessageIds">The IDs of the webhook messages to delete.</param>
-        Task DeleteMessagesBulk(IEnumerable<ulong> postedMessageIds);
+        Task DeleteMessagesBulk(IReadOnlyCollection<ulong> postedMessageIds);
         
         /// <summary>
         /// Gets the most recent message sent by a given account in a given guild.

PluralKit.Core/Services/PostgresDataStore.cs

@@ -296,7 +296,7 @@ namespace PluralKit.Core {
                     _logger.Information("Deleted message {Message}", id);
         }
 
-        public async Task DeleteMessagesBulk(IEnumerable<ulong> ids)
+        public async Task DeleteMessagesBulk(IReadOnlyCollection<ulong> ids)
         {
             using (var conn = await _conn.Obtain())
             {

PluralKit.Core/Services/ProxyCacheService.cs

@@ -187,6 +187,7 @@ namespace PluralKit.Core
         public ulong[] Accounts;
 
         public SystemGuildSettings SettingsForGuild(ulong guild) =>
+            // O(n) lookup since n is small (max ~100 in prod) and we're more constrained by memory (for a dictionary) here
             SystemGuild.FirstOrDefault(s => s.Guild == guild) ?? new SystemGuildSettings();
         
         public MemberGuildSettings SettingsForMemberGuild(int memberId, ulong guild) =>