explode has_and_belongs_to_many Rdv associations

Created by: adipasquale

this allows adding behaviour to the join tables (rdvs_users and agents_rdvs) items :

1. touching the Rdv when an association is updated or created
2. use dependent: :destroy on rdv's has_many :rdvs_users and other associations

adding primary keys to the join tables is necessary for point 2.

this PR will help storing RDV's versions for agents and users updates in #598