Delete old external students

[Petitoto]

Subject of the issue

Sometimes, we have to create an account for a non ECL student. Currently, there is no easy way to delete them later.

Your environment

• Version of Hyperion : latest main

Expected behaviour

We should add a new account_type for these accounts, and add the ability to clean these accounts periodically (we should take a decision regarding the potential content the users created (linked to their account in the database with foreign keys))