build every commit, use commit status to track progress

  - stop relying on comments & times for tracking jenkins builds
  - build every commit in a PR (using new jenkins jobs -- need to take a sha param)
  - assume we own the job when args line up, since it's building a specific commit
  - PLS REBUILD overrides all this logic, is taken into account only once
    - track which rebuild comment/command has been taken into account using a kitty-note-to-self
  - still posts a comment on failure, as github only looks at last commit to determine mergeability

review by @jsuereth, /cc @lrytz, @gkossakowski, @paulp, @jamesiry