also drop xml-code for Shareable

handle photos in StatusMessageCreationService

this creates inconsistent states, if you remove the block in the future

also refactored receive methods

* Contact: auto-follow-back
* Shareable: create share visibilities

first implementation of receive_entity callback

and do some refactorings

move it to Diaspora::Federation::Entities and use it in some tests, but most of the tests can be removed later.

closes #6877