Otherwise the mentions will be counted twice.

Otherwise, when leaving as part of RAI and in a MUC where we only get
presence info based on probes, we lose occupant info that doesn't get
recreated when we rejoin.

Also fix showing of version number in login form

This is the first commit that makes use of the `ElementView` from
Skeletor, which lets us turn views into custom elements.

When a MUC starts out hidden, we first need to join in order to find out
whether we're affiliated before we can know whether we should enable RAI
or join the MUC as usual.

To avoid a long empty flash while new messages are being fetched again.

by the user, and not as part of cleanup during logout or when leaving
automatically in order to receive XEP-0437 activity indicators.

- Add test for incoming RAI message
- Only enable RAI if the user is affilated in MUC being left
- Handle error presence indicating a resouce-constraint
- Don't unregister stanza handlers in `leave`, since we still want to
  listen to RAI-related stanzas. Instead unregister upon the `destroy`
  event.

See: https://xmpp.org/extensions/xep-0333.html#rules-muc

- Send marker for last message before leaving and before subscribing to RAI
- clear cache of RAI-subscribed domains on reconnection