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This change adds buttons to Pace Plans that allow the plan to be
reverted to the last published state (canceled) or created/updated in
the database (published). The buttons should only be active whent
there are new changes to be saved, and when no saving or loading
actions are already in progress. While the request to publish is
running, the publish status section in the upper right will show
"Publishing plan...".

NOTE: This does NOT implement actually tracking publishing progress
yet-- at this stage, the "Publishing plan..." status message only
corresponds to the publish request being in flight. Work to actually
track the delayed job status will be handled in a follow-up PS.

closes: LS-2449
flag = pace_plans

Test plan:
  - Enable Pace Plans feature flag and setting on a course with
    modules (probably just assignments for now to be safe)
  - Go to Pace Plans and expect to see a new plan with all 0s for the
    module item durations, and with cancel and publish buttons
    disabled
  - Enter a start date and add some durations, and expect the cancel
    and submit buttons to become enabled
  - Expect to see nothing displayed in the upper-right "publishing
    status" section of the screen on this new plan
  - Click the "Publish" button and expect see the buttons disable and
    a spinner with "Publishing plan..." text to appear briefly in the
    upper right
  - Enter some more duration and/or settings changes, expect to see
    the buttons enable again
  - Click "Cancel" and expect the durations and settings to reset to
    what they were when you last published the plan
  - Enter more changes and click "Publish", expect updating the plan
    to work the same as creating it
  - Ensure that switching to and from student plans still works as
    well

Change-Id: I0d654f00e575f168d78ecf9dd27e59b8cda9a8a6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/275520

Reviewed-by: Isaac Moore <isaac.moore@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Isaac Moore <isaac.moore@instructure.com>
Product-Review: Peyton Craighill <pcraighill@instructure.com>