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.

We were removing the disco entity, but it's associated collections
(`features`, `fields`, `identities` etc) were still stored in
localStorage and not cleared.

So when the entity gets recreated, the stale localStorage cached items
repopulate the collections.

Added `refreshFeatures` API method for refetching disco features and use
that instead.

`toLocaleString` now returns element attributes in alphabetical order
(for better cross-browser consistency).

Also, `toLocaleString` is now used in favor of `outerHTML` because
browsers aren't consistent with one another in their output.

Otherwise compatibility with Conversations is broken.
Unfortunately this means that OMEMO won't work on Edge, because it
for some reason requires the additionalData.

References:
* https://github.com/conversejs/converse.js/issues/1187#issuecomment-426276243
* https://stackoverflow.com/questions/42933162/web-crypto-api-using-microsoft-edge-38-14393-0-0/42950731

Fixes #1187