When inviting someone to a members-only room, you only want to add an
affiliation of 'member' if they don't already have any other affiliation.

So we need to ignore the "new" member if they're already on *any* of the lists.

Make the delta algorithm simpler. Fixed some delta bugs in the process.

One for each affiliation (member, owner, admin).

Unfortunately Prosody doesn't give you correct results if you query for all
three affiliations in one stanza.

We'll have to see whether setting all three in one stanza actually works.

As required by XEP-0045.

This requires that we first fetch the existing member list, compute the delta
and then send that if there is one.

When it's clear that a previous teardown didn't happen.
So that objects can be garbage collected and that obsolete event handlers don't
fire anymore.

It gets created anew when reconnecting or logging in.

So that it can be overridden.

Also, use sendPresence from latest strophe so that we have a timeout in case a
responding presence never comes.

- Remove apparently unused `giveFeedback` method on ControlBoxView
- Don't show old connection feedback when rendering a new login form.
  Now also no need to explicitly remove feedback after disconnection.

Due to changes in `converse-core` the controlbox wasn't aware anymore of
disconnection or reconnection events.

and when auto_login is not `true`. Will only work if the page wasn't reloaded,
because then the password will still be on the connection.

Instead, keep track of the connection state of the room and send a presence
only when the room's connection was disrupted.

Switching from bookmarks form to config form shows only spinner.

No need to manually call `renderChatArea`.

- Fixed a sessionStorage caching bug caused by putting the nickname (which is
  liable to change) in the cache key.
- Clear the occupants sessionStorage when closing a chat room.

Now that we cache disco info and not the room configuration.

Instead of the room configuration.
This is because non-owners have access to the former, but not the latter.

What started as an attempt to fix a bug in parseXUserElement, turned into
another large refactoring of MUC code, and it's not clear how to break this up
into multiple atomic commits. So I'm just pushing it all.

At least there are two new tests added to the suite.