Fix unified inbox blockers caused by ambiguous route params

* Fix ambiguous envelope/message identification
* Use envelope data to change flags instead of route params
* Fix next/prev envelope shortcut handling
* Fix saving attachments to the cloud
* Fix saving all cloud attachments
* Fix deleting envelopes (and messages)
* Fix selection of composer account

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>