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.Bot/Handlers/MessageCreated.cs

@@ -118,7 +118,7 @@ namespace PluralKit.Bot
             
             try
             {
-                await _proxy.HandleMessageAsync(evt.Client, cachedGuild, cachedAccount, msg, doAutoProxy: true);
+                await _proxy.HandleMessageAsync(evt.Client, cachedGuild, cachedAccount, msg, allowAutoproxy: true);
             }
             catch (PKError e)
             {