Switch over to django.contrib.auth.views.login
Once upon a time, django.contrib.auth didn't have a built-in login view. As of

about half a decade ago, it does. This means we can kill the one in djblets,

which isn't nearly as well maintained.
This change switches us over to the new view and form, which works pretty

similarly to the old one. The major differences are in the naming of the error

fields, next vs. next_page (which I'll deal with once I stop using

djblets.auth.util.login_required), and CSRF protection.

Tried to login with an invalid username and password and saw the right error.
Tried to login after fiddling with the CSRF token and saw a 403.
Logged in with a valid username and password and was redirected to

  /dashboard/
Checked that when I accessed a page when logged out I was redirected to log

  in, and then once I logged in, I was redirected back to that page.