Fix review request ID in email subjects.

We were using the id in email subjects instead of display_id. Very simple fix.

Reviewed at http://reviews.reviewboard.org/r/4032/
Fix errors with the use_ticket_auth form field.

When saving repositories, we can sometimes hit an issue where get_scmtool()
fails due to the repository object not actually existing. Since we don't really
care about having this in extra_data for repositories which don't support ticket
auth, we don't need to include this check.
Fixed overzealous initialization of PYTHONPATH.

This is a bugfix over __init__.py that was inititalliy changed at
https://github.com/reviewboard/reviewboard/commit/915452d483a812ee34006741a6f5c23a4d03a5b1

The current initialization is overzealous and conflicts with the
installation call from extensionbrowser's easy_install usage (with the
current initialization, an easy_install call would nuke all entries in
the easy_install.pth file and overwrite previous entries).

Reviewed at http://reviews.reviewboard.org/r/4027/