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

and turn them into a regular string.

Also removes support for anonymous forum posts.
Closes #698

to work-around android bug #224270.
This is only necessary if the transitioning view might not be available
anymore when the exit transition is made.