Made staff_member_required redirect to login

Refs #21911.
Fixed #21911 -- Made admin views redirect to login when needed

Historically, the Django admin used to pass through the request
from an unauthorized access to the login view directly. Now we
are using a proper redirection, which is also preventing
inadvertantly changing data when POSTing login data to an admin
view when user is already authorized.
Thanks Marc Tamlyn and Tim Graham for the reviews.
Removed the this_is_the_login_form hack

Refs #21911. Now that we have a more traditional login form, we
don't need any more a special field telling us we are dealing with
the login form.