Switch to django's built-in login_required decorator.
There's no longer any reason for us to have our own login_required decorator,

and it's just more code for us to maintain. The upstream django one provides

everything we need (plus some nice checking that we didn't have).
This is a little bit ugly because of the name of the 'next_page' field on the

login page, but that will be a trivial cleanup once this goes in.

Checked that all the views which explicitly used login_required and a few of

the views that used check_login_required correctly redirected to the login page

with next_page set properly. Once there, checked that logging in redirected to

the correct page.