[Mod] Check for filtered words on message edit

As proposed in #767

cogs/mod.py

@@ -1595,6 +1595,18 @@ class Mod:
         if not deleted:
             deleted = await self.check_mention_spam(message)
 
+    async def on_message_edit(self, _, message):
+        author = message.author
+        if message.server is None or self.bot.user == author:
+            return
+
+        valid_user = isinstance(author, discord.Member) and not author.bot
+
+        if not valid_user or self.is_mod_or_superior(message):
+            return
+
+        await self.check_filter(message)
+
     async def on_member_ban(self, member):
         server = member.server
         if not self.temp_cache.check(member, server, "BAN"):