Do not add a space after /unignore completion
Add an indicator of the number of participants

(like in weechat)
Fix a potential traceback on /ignore

(if the room was not joined)
Fix the background color of some messages
Improve XEP-0308 support

- Prevent correction of delayed messages
- Prevent correction of messages by someone else in a MUC (and in a
  private tab)
- Messages with unauthorized corrections (above) or wrong message id
  will be displayed as normal messages

TODO: restrict the corrections to the same fullJID (only in direct
"normal" conversations, because we can know in private an muc tabs, via
the User object)