Make admin user accounts the primary way of accessing admin interface

[sebbacon]

Currently, the admin UI is protected by Basic HTTP Auth, either via Apache config, or via the ADMIN_USERNAME credentials in the Alaveteli config,

There are some admin UI elements and tasks in the frontend which are separately protected by a "superuser" role for ordinary user accounts.

These two authorisation schemes should be unified.

I propose:

• The "superuser" role becomes the primary method for authorisation all admin activity
• We retain the ADMIN_USERNAME account as a superuser (or emergency user) -- one that is able to create new user accounts etc
• For backwards compatibility, it should still be possible for existing (i.e. known) accounts to log in via Basic HTTP Auth; however, it should be enforced that new admin accounts must always use the "superuser" role

See also issue #73 and #34 (and perhaps #209), and this mailing list thread for context.