rb-site upgrade now checks whether the htdocs/reviewboard.wsgi file

makes use of an older WSGI loader from either Django 1.6 or 1.11. If so,

it's upgraded to use the new reviewboard.wsgi module, stripping out

some of the old environment settings in the process, and adding the

REVIEWBOARD_SITEDIR setting. This will help keep things maintainable,

and avoid issues later as requirements or configuration needs change.

Unit tests pass.
Manually tested upgrading sites from a 3.0 and a 4.0 beta 2 install and

seeing the new WSGI files in both cases. Verified the new files worked.