For example `This groupchat is not anonymous`

Mostly to match any previous leave/join message from the same user

Fixes #1062

And fix an HTML rendering bug for info messages and nicks that contain spaces

Previously we checked only if the last message was a join message from
the same person.

Now instead we check the last n messages that are join or leave
notifications.

* Don't show leave notification twice when someone leaves, joins and then leaves again
* Add a test case for a member joining and then leaving

Fixes #1169

Seems to make some sense after all, but we need to add a check that the
user isn't a roster contact as well.

The second especially doesn't make sense and I'm currently not aware of
a case headline stanzas with a JIDs containing `@` and with a body
exist.

Updates #1221

if we don't have anything to show inside it.

Updates #1221

* Typo in error message

Add config setting `send_chat_state_notifications`

Hide "add contact" button and requesting/pending contacts when
allow_contact_requests is false.