Fix the display of static media upgrade instructions in rb-site.

The static media upgrade instructions in rb-site didn't reliably work,
due to the state of the database not being what was expected at the time
when we output the instructions. So now we do the check before the
upgrade process begins.
Disallow installing on older versions of Python.

setup.py now checks if the version of Python on the system is new enough
for running Review Board. This should prevent the issue of someone
trying to upgrade or install on Python 2.4, getting the source tarball,
and then failing with confusing errors.
Add support for moved file handling for Perforce.

This adds support for representing moved files properly in Perforce.
This handles both files moved with and without changes made.

An accompanying change to post-review is needed in order to add moved
information to the diff format. Like the other fields in Perforce diffs
we respond to, this is entirely custom, as Perforce doesn't have any
useful information in p4 diff to provide.

This feature is funded by NetApp, Inc.