Define initial celery task on instructor dash.

Add CourseTaskLog, and make calls to regrading.

Add regrading implementation, and hack the marshalling of request.
Add task progress table to instructor dash.

Add call to MakoMiddleware() to initialize templates in celery worker server.

Pass task_ids by POST properties (as a list) to collect task progress status.
Pull task_queue.py methods out from tasks.py, to represent API calls
from client.  Tasks.py remains the task implementations running on the
celery worker.

In particular, move status message generation out of task thread to client side.
Refactor tracking and xqueue initialization to no longer require a request.  Get tracking logs to work.  Add transaction annotations.

Make sure to read only once from result objects when getting result status,
so that values are consistent.
Add additional background tasks:  reset attempts, delete state.

Update CourseTaskLog fully after task submission, so it works in Eager mode (for testing).
Clean up task progress javascript.  Add before/after scores to tracking of regrading requests.
Add tests for regrading at capa_module level.
Add check for problems that (do not) support regrading.
Check descriptor to identify problems that don't support regrading.
Add factory for CourseTaskLog.

Add unit tests for regrading at courseware level (task and task_queue).
Remove delete button from instructor dash. (Also remove last-modified
column from task-in-progress table).
Add background task history to instructor dash (as table).

Add task-history-per-student button, and fix display of task messages.
Add duration to task status.  Add tests for reset-attempts.
Post-rebasing fixes to re-enable masquerading.
Define MITX_FEATURES['ENABLE_COURSE_BACKGROUND_TASKS'] to allow background tasks to be removed from instructor dash.
remove call to psychometrics when regrading
Add tracking calls to reset (and delete).  Add tests for delete.  Remove sleep statement used for manual testing.
Add unit test to regrade problem using randomization.
Disable randomized-problem test until bug-fix is merged in.
fix some pylint/pep8