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.

Fix relayable retraction sender

use the parent author as salmon sender, if the parent author is local

Federation rewrite with diaspora_federation gem

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

send retraction for relayable to target author if retracted by parent author

* remove author_signature column
* delete all self-participating participations
* delete all participations where author and post-author are remote
* delete all duplicate participations
* delete all participations without post
* create unique index

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