Add code supporting CMS labelling

Someone had mentioned to me in the last couple of weeks that you can't filter pull requests in GitHub based on the target branch.  So, I've gone ahead and written code to do that.

This does two things:
1) Adds a CLI script which can update all open PRs for a project (coded to only support the CMS for now)
2) Adds code to the pull requests hook to add the appropriate label to GitHub (again, coded only to support the CMS for now)