This updates the generated htdocs/reviewboard.wsgi file to include the

virtualenv Python path, which will be used if a virtualenv is set up.

This is needed in order to make the reviewboard import work correctly.

The main reviewboard/wsgi.py will then proceed with full virtualenv

activation.
Existing copies of reviewboard.wsgi will be upgraded with the new

code. The upgrade logic has grown more complex in time, and as part of

this change, it's been overhauled to be more maintainable. We now try

to determine custom settings added to the script by admins and preserve

those, but otherwise rewrite the file to a completely custom version.
This mostly simplifies the upgrade logic, but the way we handle upgrades

does depend on what we're upgrading.
For older (pre-5.0.6) files, we now remove everything we've ever added

from the file and use the rest to determine any custom settings from the

administrators, which we'll preserve.
For 5.0.6+ installs, we look explicitly for a commented region

containing custom settings (wrapped by # BEGIN CUSTOM SETTINGS and

# END CUSTOM SETTINGS).
We then output the final script based on the template file.
Along with these changes, we no longer import the imp module in

rb-site. This isn't available on newer installs, and our usage of it

wasn't really necessary.

Tested generating a new site and verifying the resulting script worked

and resulted in an activated virtualenv with Review Board and Django.
Tested upgrading some versions of older and custom-altered

reviewboard.wsgi files.
Unit tests pass.