Setting it on the model itself is not sufficient and then causes
changes which should be type `update` become type `create`, causing
multiple versions of the model... resulting in chaos.

Otherwise the collection's items aren't fetched but it's own properties.
The way to persistently remove items from a collection is to destroy
them, NOT to call `remove`.

Based on live testing. Updated tests accordingly.

- Use sizzle xmlns selector (which doesn't work with `querySelector`)
- PEP Subscription didn't have `+` before `notify`

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