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.

Send a second message to remote recipients with conversation-author
as salmon author if the conversation-author is local.

The first dispatch will fail on old pods. New pods will ignore the
second message, if the guid already exist.

the check that the participation parent author is local is now done in
the diaspora_federation gem

only deliver to parent author

message doesn't include Diaspora::Relayable anymore

and delete old batch worker and receiver

also drop xml-code for Shareable

handle photos in StatusMessageCreationService

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