Add admin authentication

[Hubro]

Add optional admin authentication using the admin_password config key.

If provided, this password will unlock administrative actions in Clipface, like uploading/renaming/deleting clips and adding/editing clip titles and descriptions.

Admin authentication will use the same login box as the regular user authentication.

If user authentication is enabled, then enabling admin authentication will not make any visible difference to the user interface. If the admin password is entered in the password box, the user will be logged in as admin. If already logged in as a regular user, the user must log back out and then log in as the admin user.

If user authentication is not enabled but admin authentication is enabled, a new button will be added in the header called "Log in" to allow the user to log in as admin.