Fix the display of draft comments in review UIs.

Draft comments from other users were being shown in review UIs, since we
weren't filtering out based on whether the current user had the ability
to see the comment.

This affected the image review UI, and anything else using the default
get_serialized_comments function.
Relax markdown version requirement.

The minimum markdown version requirement for Python 2.7 is now 2.2.1,
which will make it easier for certain setups using older versions to
upgrade. This was an issue for OpenShift.

Reviewed at https://reviews.reviewboard.org/r/4359/
Add a X-ReviewRequest-Repository header to e-mails.

This adds a X-ReviewRequest-Repository header to all outgoing e-mails.
This can help assist in filtering of e-mails when dealing with many
different repositories on Review Board.

Based on https://reviews.reviewboard.org/r/4328/, but with the header
renamed.