fix(nb_versions): coherence

* Added signal to catch version deletion and make sure nb_version in versioned models is coherent
* Added test for this
* Made sure pending moderation data (that was automatic) and versions are deleted when a model is deleted.
* Added test for this.

Fixes #84
6 jobs for !78 with fix/coherentBackend in 5 minutes and 11 seconds (queued for 2 seconds)
merge request