rb-site upgrade has logic for migrating reviewboard.wsgi from older

versions of Review Board to a newer format. As part of this, we remove

certain lines, including one that sets DJANGO_SETTINGS_MODULE. This

would remove any line setting this, but if a site had some specific

needs and needed to point to a custom module, that would be removed

during upgrade.
We now only remove the versions that set reviewboard.settings as the

module, preserving anything more custom.
This won't affect nearly any users but us.

Verified this preserved a custom module set in reviewboard.wsgi.