Add `no_users` check to login http endpoint

[Caleb Ellis]

Before the UI split, the login template would conditionally display init information if there were currently no MAAS users:
'No admin user has been created yet. Use the "init" administration command to create one.'

The login response used to include `'no_users': UserProfile.objects.all_users().count() == 0` but doesn't anymore. We should add this back so we can add the warning to the template.

This should also be backported to 2.7.

GitHub issue: https://github.com/canonical-web-and-design/maas-ui/issues/969