feat: grant staff access to superusers automatically

[Abdul-Muqadim-Arbisoft]

Description

Django's is_superuser and is_staff are independent flags. A superuser bypasses Django's permission checks, but it is is_staff that actually gates the Django admin and a number of staff-only Studio/LMS views. As a result, a superuser can end up unable to reach the admin or those views until someone also sets is_staff, which is confusing and offers no real protection, since a superuser can grant itself is_staff at any time.

This change makes every superuser a staff user automatically:

• pre_save signal (common/djangoapps/student/signals/receivers.py), grant_staff_access_to_superusers sets is_staff = True on any user being saved with is_superuser = True. It runs before the DB write, so there is no
  extra query and no signal recursion. Fixture (raw) loads are skipped.
• Data migration (student/migrations/0050_grant_staff_to_existing_superusers.py), backfills existing superusers (is_superuser=True, is_staff=False → is_staff=True) in a single bulk update, since the signal only affects future saves. The
  reverse operation is intentionally a no-op.

Behavior is one-directional by design: it grants staff to superusers but does not remove staff when a user stops being a superuser.

Supporting information

Forum discussion: https://discuss.openedx.org/t/shouldnt-superuser-automatically-inherit-staff-access-in-open-edx/18657

Feanil suggested "a small signal handler or something that would just grant all superusers the staff role as well", this implements exactly that, plus a one-time backfill for existing installs.

Testing instructions

1. Create a superuser via the Django admin or shell with is_superuser=True and is_staff=False, save it, and confirm is_staff is now True.
2. Promote an existing non-staff user to superuser and confirm they gain staff access.
3. Confirm a regular (non-superuser) user's is_staff value is left untouched.
4. On an existing install with a non-staff superuser, run migrations and confirm that superuser becomes staff.