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/Proxy/Autoproxier.cs

@@ -0,0 +1,93 @@
+#nullable enable
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+
+using NodaTime;
+
+using PluralKit.Core;
+
+
+namespace PluralKit.Bot
+{
+    public class Autoproxier
+    {
+        public static readonly string EscapeString = @"\"; 
+        public static readonly Duration AutoproxyExpiryTime = Duration.FromHours(6);
+
+        private IClock _clock;
+        private IDataStore _data;
+        
+        public Autoproxier(IDataStore data, IClock clock)
+        {
+            _data = data;
+            _clock = clock;
+        }
+
+        public async ValueTask<ProxyMatch?> TryAutoproxy(AutoproxyContext ctx)
+        {
+            if (IsEscaped(ctx.Content))
+                return null;
+
+            var member = await FindAutoproxyMember(ctx);
+            if (member == null) return null;
+            
+            return new ProxyMatch
+            {
+                Content = ctx.Content,
+                Member = member,
+                ProxyTags = ProxyTagsFor(member)
+            };
+        }
+        
+        private async ValueTask<PKMember?> FindAutoproxyMember(AutoproxyContext ctx)
+        {
+            switch (ctx.Mode)
+            {
+                case AutoproxyMode.Off:
+                    return null;
+                
+                case AutoproxyMode.Front:
+                    return await _data.GetFirstFronter(ctx.Account.System);
+                
+                case AutoproxyMode.Latch:
+                    // Latch mode: find last proxied message, use *that* member
+                    var msg = await _data.GetLastMessageInGuild(ctx.SenderId, ctx.GuildId);
+                    if (msg == null) return null; // No message found
+
+                    // If the message is older than 6 hours, ignore it and force the sender to "refresh" a proxy
+                    // This can be revised in the future, it's a preliminary value.
+                    var timestamp = DiscordUtils.SnowflakeToInstant(msg.Message.Mid);
+                    if (_clock.GetCurrentInstant() - timestamp > AutoproxyExpiryTime) return null;
+                    
+                    return msg.Member;
+                
+                case AutoproxyMode.Member:
+                    // We already have the member list cached, so:
+                    // O(n) lookup since n is small (max 1500 de jure) and we're more constrained by memory (for a dictionary) here
+                    return ctx.Account.Members.FirstOrDefault(m => m.Id == ctx.AutoproxyMember);
+                
+                default: 
+                    throw new ArgumentOutOfRangeException($"Unknown autoproxy mode {ctx.Mode}");
+            }
+        }
+
+        private ProxyTag? ProxyTagsFor(PKMember member)
+        {
+            if (member.ProxyTags.Count == 0) return null;
+            return member.ProxyTags.First();
+        }
+
+        private bool IsEscaped(string message) => message.TrimStart().StartsWith(EscapeString);
+        
+        public struct AutoproxyContext
+        {
+            public CachedAccount Account;
+            public string Content;
+            public AutoproxyMode Mode;
+            public int? AutoproxyMember;
+            public ulong SenderId;
+            public ulong GuildId;
+        }
+    }
+}