Sometimes users end up in situations where they're running Review Board
in the web server using a different Python version than they're now
using on the command line, or they're using a mix of eggs and wheels.

We now display the Python version and the install path in
`rb-site --version`, and we include this information along with some
possible resolutions in the Version Mismatch page. This should help get
people unstuck, and give us more tools for debugging when things go
wrong.

Testing Done:
Tested the version mismatch and saw the Python version and install path
being used.

Ran `rb-site --version` and saw the Python version and install path.

Reviewed at https://reviews.reviewboard.org/r/9025/