GitLab

Updates Pace Plans to check for the status of delayed publishing
jobs on a continual basis to know when publishing is done for real.

closes LS-2759
flag = pace_plans

Test plan:
  - Enable Pace Plans feature flag and course settings
  - Make sure you have delayed_job running
  - Go to a course with a new or existing Pace Plan
  - Make some changes to the pace plan and click "Publish"
  - Expect to see the publishing status in the upper-right part of the
    screen show "Publishing plan..."
  - In the Dev Tools network tab, check to see that requests are being
    made to the /api/v1/progress/<id> every 3 seconds
  - Expect the status to change to "All changes published" once the
    delayed job has finished

  - Do something to introduce delay into the delayed job (set a
    breakpoint, add a sleep, etc...)
  - Make some more changes to the plan and click "Publish" again
  - Refresh the page to reload Pace Plans
  - Expect the publishing status to be "Publ...