* Fix issue #6847

Previously the federation layer has written the empty string to the
database, now it writes nil/no value to it while our code still expects
the empty string in some cases. Restore the old assumption by returning
the empty string, the real raw value by now is available again in
StatusMessage#text for those that need it, such as validations or tests.

only deliver to parent author

message doesn't include Diaspora::Relayable anymore

also refactored relayable specs to use `let`

and delete old batch worker and receiver

also drop xml-code for Shareable

handle photos in StatusMessageCreationService

also refactored receive methods

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

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.

No uniqueness control on AspectVisibility resulted in possible having
multiple AspectVisibility objects in the DB for the same aspect and
shareable which doesn't make sense. Introduce uniqueness validation
and fix up tests where duplication happened.

Marking a notification as unread resets the timeago stamp causing
the times to look wrong. It can be reproduced by marking an old
notification as unread. Using the update_column instead of
update_attribute will not touch the updated_at attribute, and
thus will not affect the updated time ago in the view.

Fixes #6798.

closes #6821

* move presenters back to controllers, this is view-logic
* use PostService in CommentService
* remove iframe route, this is not used anymore
* id/guid limit at 16 chars, hex(8) is 16 chars long