Bounce message corrections

On IRC it is confusing when an XMPP user "corrects" a message, but on
the other side a new message is sent. To prevent spam and confusion, do
not allow message corrections at all, and instead return an error if
attempted.

Fixes: #3436
Fixes: #3480

CHANGELOG.rst

@@ -9,6 +9,8 @@ For users
   can still use the in-room JID (#chan%irc@biboumi/NickName) to send a
   private message but the response you will receive will come from
   nickname%irc@biboumi.
+- Message Correction (XEP-0308) no longer works, and will instead return an
+  error when the user attempts to.
 
 For admins
 ----------