User's role field needs to have a cascade delete

[MarkKoz]

#357 replaced the FK with an ArrayField. This introduced a problem of the roles table falling out of sync with the users table. I encountered an issue when trying to sync a user with the bot. One of their roles had been deleted from the roles table, but it was still present in their roles field. The validator ensures all roles exist in the table, so the request was failing.

Apparently Django Postgres Extensions have a solution for this.

[MarkKoz]

Django Postgres Extensions doesn't seem to be maintained any more and may be incompatible with Django 3.0.

[D0rs4n]

I believe, as a temporary workaround (until the migration to FastAPI) this could be solved by using a pre_delete signal, that unassigns the Role assigned to the User's ArrayField, then performs the process as usual.
(That's purely a suggestion, I faced the same issue in a previous code jam)