PR monitoring and status updates

[msarahan]

Right now, we use https://github.com/jtarchie/pullrequest-resource to monitor new PRs and build them. This also updates the status. However, there are some issues:

1. Only works on new PRs (ones without a concourse-ci status message). New commits on a PR are not built. Closing and reopening a PR does not trigger rebuild.
2. With the job actually split between determining builds and then carrying out builds, the PR status update must be forwarded somehow to the latter of those two pipelines, while the triggering must be done on the former.