Rename fields in CourseTaskLog, including a task_key column for indexing.  Use 'rescore' instead of 'regrade'.  Clean up task submission.

Add tracking calls to reset (and delete).  Add tests for delete.  Remove sleep statement used for manual testing.

Define MITX_FEATURES['ENABLE_COURSE_BACKGROUND_TASKS'] to allow background tasks to be removed from instructor dash.

Add task-history-per-student button, and fix display of task messages.

column from task-in-progress table).

Add unit tests for regrading at courseware level (task and task_queue).

CourseTaskLog fully after task submission, so it works in Eager mode
(for testing).

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.

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.

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.

Add CourseTaskLog, and make calls to regrading.

Add regrading implementation, and hack the marshalling of request.

Jonahstanley/add navigation tests

Anton/videoalpha tests