More complete support for onboarding new registrars using the Django admin

[rebeccacremona]

Our codebase presumes that new registrars are created/approved via the GUI at https://perma.cc/manage/registrars, and that new registrar users will be associated with them via the GUI at https://perma.cc/manage/registrar-users.

But it is often more convenient for Perma's admins to add new registrars and registrar users via the Django admin. They can... but it doesn't work perfectly as is. For example, any newly-created registrar users aren't sent the email they need to activate their account. (There may be other subtle difference as well: to be investigated!)

Let's update the Django admin such that adding/approving registrars and adding registrar users works just as well. (Which will involve refactoring the code in https://github.com/harvard-lil/perma/blob/develop/perma_web/perma/views/user_management.py).

More Context

The "Approve" Button

When an admin approves a registrar's application (a sign up request from https://perma.cc/libraries, for instance) via the approve button at https://perma.cc/manage/registrars/approve/, screenshot above, this logic is run.

The "Add Registrar" Button

When an admin creates a new registrar from scratch using the "add registrar" button at https://perma.cc/manage/registrars, screenshot above, this less complex logic is run.

The "Add Registrar User" Button

When an admin adds a new registrar user to a registrar via the button at https://perma.cc/manage/registrar-users, screenshot above, this logic is run (via this view, a subclass, registered here.

The Django Admin

In the Django admin, new registrar users are being added via this inline form. on the RegistrarAdmin