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
Status Job ID Name Coverage
  Check
passed #481894
docker
check_back

00:00:25

passed #481895
docker
check_front

00:01:18

 
  Test
passed #481896
docker
test_back

00:00:45

87.0%
passed #481897
docker
test_frontend

00:00:58

 
  Lint
passed #481899
docker
eslint

00:01:00

passed #481898
docker
flake8

00:00:13

 
  Svg Gen Docu
passed #481900
docker
generate_UML_svg

00:00:13

 
  Documentation
passed #481901
docker
pages

00:00:12

 
  Deploy
passed pages:deploy

00:00:00