Beta invite sites framework

This contains one fix and one improvement:

(1) Uses `Site.objects.get_current()` to build the invitation verify url when sending emails. This is done in a similar fashion to how django-registration does it.

This is needed if the admin site and the production site have different domains.

(2) I found an old reference to the named url pattern `beta_invite_url` (that's the only occurrence, I grepped the whole repo), which I updated.