Make `is_staff` true when a user is Staff Admin (#3841)

Fixes #3782.

Rather than having `is_staff` set to true when a user's
email is the org's domain, this sets it true when a user is Staff Admin
or a Superuser. Also includes a migration to handle existing cases where
Staff Admins are not `is_staff`.

Co-authored-by: Sandeep Chauhan <sandeepsajan0@gmail.com>
Co-authored-by: Saurabh Kumar <theskumar@users.noreply.github.com>