Fix next/next_page mess.

For representing the page to redirect to, the djblets auth code used
'next_page', while django's auth used 'next'. Now that the login view and
decorators are entirely converted to use django's, we can convert these to use
'next'.

There's still one instance of 'next_page' used in the registration page. I
haven't yet looked to see how much of that page can be replaced with code
from django.contrib.auth.

Testing done:
Verified that all the changed locations redirected to the login form with the
correct variable names set, and that logging in redirected to that page.

Reviewed at https://reviews.reviewboard.org/r/4933/