As part of an effort to move toward encouraging virtualenv-based

management of Review Board installs, this change updates our WSGI

entrypoint and our auto-generated configurations to detect and

sanity-check a virtualenv setup and activate it if available.
reviewboard/wsgi.py now looks to see if we have a $sitedir/venv

directory. If it exists, and we're not already in a virtualenv, it will

make sure the right files are present and then activate it. If anything

is missing, a RuntimeError will be raised with instructions, helping

admins sort out the issue.
Validation checks for the presence of an activate_this.py (which is

created by virtualenv but not python -m venv) and a pythonX.Y

binary matching the currently-executing Python version.
Environment variables are built from the new state, ensuring

subprocesses will see the correct versions of binaries and packages.
This also adds a $sitedir/bin to $PATH, giving us a place to put

executables like p4 without placing them in system directories.

Tested with both a global Python and a virtualenv-backed install.

Verified that the venv directory was present for a virtualenv-backed

install, and that its environment was properly used in a default Apache,

uWSGI, and Gunicorn setup.
Tested pruning some of the virtualenv dir to hit the WSGI error checks.

Verified the resulting errors.