[C2] Remove ADMIN vs SUPERADMIN redundancy

[ywarnier]

In C2 alpha, 3 admin roles are created:

• global admin (an admin in the context of multi-urls)
• superadmin (a local admin that can use "login as")
• admin (a local admin without "login as")

This single change between superadmin and admin is not worth the confusion.
Add a migration to remove the "Superadmin" role and add the "login as" permission to "admin" (or remove "admin" and change the name of "superadmin" to "admin" :-)).

[christianbeeznest]

It is done with this PR #6774

Thanks for confirmation.

[ywarnier]

A simple check shows that the SUPERADMIN role has been removed. This is great. Thanks!