Display more info in rb-site --version and the Version Mismatch page.

Review Request #728 — Created June 21, 2017 and updated — Latest diff uploaded

Review Board
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

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/