Users can now be "superusers"

Also changed it so that only superusers can edit applications.

The intention is that we'll use this role for other things - for
example we'll move from an admin being able to make/unmake people as
admins to that being restricted to superusers.
Let superusers edit most attrs of applications

Also add some help text and improve UX.
Explain what different roles are able to do
Fix whitespace
Only superusers can set user roles
Merge remote branch 'origin/master' into add_superuser_role

Conflicts:
	app/views/superuser/applications/index.html.erb
Fix a path that has changed
Rename superuser to superadmin

Superuser might imply a user who isn't quite an admin but is more than a user,
but actually they are more than an admin.
Merge remote branch 'origin/master' into add_superuser_role

Conflicts:
	app/views/admin/applications/edit.html.erb
	config/routes.rb
Merge pull request #45 from alphagov/add_superuser_role

Add superuser role