[service] Absract::get: adapt for publishing model

A conference abstract is published, if the abstract
is accepted and the conference is published.
[test] adapt for changed publishing model

Addapt tests to check for Accepted instead of Published
[test] assets: abstract[0] Published -> Accepted
[models] remove AbstractState.Published

This state is not needed anymore; we store strings in the
DB so removing this should be fine as long as we don't
have an "published" abstract in the DB.