Also, declare it in the controlbox plugin, not in core, where it's not used.

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).