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.

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.