Add complete historisation of all the cv entries

This MR uses the simple history plugin for django:

https://django-simple-history.readthedocs.io

We add it here to keep track of every change done in the CV, so that we could take a look on how the terms evolved.

The MR currently doesn't contain any UI change - we may can add this in the future. But all of changes are then saved in the db, so we can take a look there - or via the manage.py shell command and the .history field of the model entities.