Allow any linked account to edit/delete messages sent by the same system

Closes #375.

PluralKit.Bot/Handlers/ReactionAdded.cs

@@ -115,8 +115,11 @@ namespace PluralKit.Bot
             if (!_bot.PermissionsIn(evt.ChannelId).HasFlag(PermissionSet.ManageMessages))
                 return;
             
+            using var conn = await _db.Obtain();
+            var system = await _repo.GetSystemByAccount(conn, evt.UserId);
+
             // Can only delete your own message
-            if (msg.Message.Sender != evt.UserId) return;
+            if (msg.System.Id != system.Id) return;
 
             try
             {