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

column from task-in-progress table).

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

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.

- Remove single enrollment/unenrollment
- Make unenrollment work with multi-email listbox.
- Provide checkbox for auto-enroll on activate if user not yet signed
up.
- Enroll student when activating if auto-enroll flag set.

this makes it possible to reset open ended response problems

Taking Ike's delete-student-state-for-problem code and putting it in a separate PR so it's not bundled with more debated features.

- lets user see and add cohorts
- needs styling
- needs to allow them to actually manage membership!

Replace all calls to analytics service to use the get methods (pulls from mongodb). Added error handling.

Added instructor tools for resetting attempts on a single student's problem to 0 and for getting student's progress page

remaining:
 * support putting multiple names / emails in the big box we now have

* still needs to support uploading a list
* also want to clean up and refactor the code a bit