Instead of checking if there's a scrollbar. Otherwise the filter disappears
when all groups are collapsed.

We don't need them at that time and we'll query when a chat room gets opened.

* `disco#items` are now only fetched when advertised by the entity.
* `identity` information is now also stored on the `DiscoEntity` model.

Listen for the `afterMessagesFetched` event.
Wait until `discoInitialized` before fetching.
Add `fetchArchivedMessagesIfNecessary` method(s).

'reconnected' cannot be used as a promise (because it's recurring),
so to avoid confusion let's not have a 'connected' promise either.

We can now support feature discovery for multiple entities (although we
currently still only query for the user's own XMPP server).

Also made some styling changes and tweaked the room head to better
differentiate between the Name and JID of a room.

so that slider does not have fixed height after being opened and can therefore
shrink or expand appropriately when its children change.

As a result we can again show emoji categories and skin tones on the same line
(if there is enough space).