Disallow correcting historical MUC messages, as the XEP-308 requires.
Previous changes introduce problem with
"Illicit LMC attempt from conference@server/user for message from user"
During investigation it was revealed that XEP does not recommend support
of historical MUC messages correction.
```
When used in a Multi-User Chat (XEP-0045) context,
corrections must not be allowed (by the receiver)
for messages received before the sender joined the room -
particularly a full JID leaving the room
then rejoining and correcting a message SHOULD be disallowed,
as the entity behind the full JID in the MUC may have changed.
```
https://xmpp.org/extensions/xep-0308.html#rules
XEP details mentioned by @jubalh
Bug discovered and solution improved by @jaeckel
a671cff964