add commtrack login flow

this adds a commtrack-specific login page (including for after you have confirmed a domain but are not yet logged in) using an optional parameter in the urlpattern, i.e. accounts/login/commtrack (in the same way as the existing user and domain registration pages) and ensures that CommTrack appears instead of CommCare HQ and the correct domain type is preserved in any navigation links throughout the registration and login processes.

I didn't completely process the fact that we're using commtrack.org as an entry point for HQ.  We should probably switch to using the django sites framework using the domain name to determine the site for these pages where a logged-in domain isn't present, instead of passing around the domain type in the URLs.  But this will work fine for now.

Tested this pretty extensively by going through every page for both types.