Missing ForeignKeys

[jgribonvald]

ForeignKeys are missing.
Such keys permit to keep relations of datas between tables and avoid problems when a data is deleted on one table and that need to remove all other datas linked.

As exemple if we remove a user, this should remove all his linked datas, we can add a on delete cascade to automatically delete linked datas.

An exemple of bug is if we delete a user it's doesn't remove entries on table credential_group_member