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

also drop xml-code for Shareable