Provide access to the course_id directly on XModules/Descriptors

Add upsell for verified certs on the dashboard.

Fixes [BLD-456] and [LMS-1392], but making sure that we're always asking
XModules about their course_id (since the ModuleSystem has access to
that data).

LMS-1127

LMS-1127

LMS-1127

LMS-1127

LMS-1127

Also includes some tests

Story: LMS-1127

Ensure that peer grading modules whose linked problem is removed change ...

Disable problem editor test

Added a [code] block to the problem markdown parser

Remove the use of celery.group from bulk email subtasks.

Improve forums performance

Avoid recomputing course module information for every thread, which
should dramatically improve the performance of high-percentile latency
queries.

JIRA: FOR-250

The _DISCUSSIONINFO global was originally used as a cache, but has since
lost that capability and is therefore just harmful. This is a precursor
to more refactoring that will improve the performance of the forums and
may itself provide some performance improvement because it separates the
computation done by two functions that each previously computed the
entirety of _DISCUSSIONINFO.

Improved release script

Allow the export_course command to dump results to stdout

This is in preparation for significant refactoring of the code in
question.

Improve forum error handling