Enable foreign key support for sqlite

[stefanoborini]

Currently our orm data model, when relying on sqlite, does not enforce cascade delete or foreign keys. The problem is that by default sqlite does not enforce foreign keys.

We need to check the docs

https://www.sqlite.org/foreignkeys.html#fk_basics

and the sqlalchemy requirements, then modify our datamodel sqlalchemy description so that cascade delete and foreign keys are enforced (e.g. when a user or application is deleted, the appropriate entries in the application policy table must be deleted as well)