This changes the visibility of some methods, removes unnecessary
abstractions and fixes static import of GroupId constant.

Also add NotNull annotation to classes that were touched

This also adds two integration tests and improves some small details

Also react to incoming group invitations

Also close keyboard when returning from message fragment

* Improve wording so contacts know they need meet up to scan
  and scan each other's codes
* Use consistent progress bar styles

added a cache to the IdentityManager, changed its signature, modified when and where the author is stored

made the author creation single-threaded again in the LifecycleManager, removed redundant code