Allow for devices to be removed.

It's usually not necessary to wrap a promise inside another one, instead
the original promise can just be returned.

under non-OMEMO circumstances. Otherwise, when messages are fetched in
bulk via MAM, then a message referring to a previous one (e.g.
a correction) may be processed before the message being referred to has
been created.

Also, fix all broken tests, mostly related to this.

Also remove Converse from fullscreen background