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/
Merge branch 'release-1.7.x'
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.
Merge branch 'release-1.7.x'
Fix cancelling of new diff comments.

Cancelling new diff comments was broken due to some of the earlier re-jiggering
of the destroy() callbacks. In the case of new diff comments when we don't have
a Review object, we were triggering the error callback instead of actually
destroying the object. I've fixed this case and added a big comment.

While I was doing this, I noticed that CommentEditor._updateFromComment was
being triggered during the cancel operation. I've made it remove that event
binding.

Testing done:
- Checked that cancelling the addition of a new diff comment with no existing
  Review would remove the comment flag.
- Checked that cancelling the addition of a new diff comment with an existing
  Review would remove the comment flag but leave the rest of the Review
  unaffected.
- Checked that cancelling a change to an existing diff comment left the existing
  comment text the same.

Reviewed at http://reviews.reviewboard.org/r/4037/
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/
Merge branch 'release-1.7.x'