Fix a bug in diff-cover

Feature/brian/regrade celery 2

Cherry pick change to url matching regular expression. Fixes LMS-37

remove assets.js from the PIPELINE_JS since it seems to cause that file ...

When changing variable names for code review critique, accidental cut/paste prevented intended behavior.

Fix a few remaining tz naive dates

need to import os

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).