Add mgmt cmd to generate anonymized ID mapping

So that instructors have easy access to the mapping from anonymized
IDs (a simple MD5 hash of the user ID that's used when integrating
with third-party services like Qualtrics) to user IDs, we have a
simple Django management command to generate a CSV mapping. To run,
use the following:

rake django-admin[anonymized_id_mapping,<system>,<env>,<course_id>]

And, of course, substitute the appropriate values for <system>, <env>,
and <course_id>. (E.g., lms, dev, and MITx/6.002x/Circuits)
Anonymized ID mapping fixups

Fixups to Nate's anonymized id mapper, repairing pep8 and pylint errors,
and rebasing on recent master.
give some debug message regarding why export might fail

Conflicts:

	common/lib/xmodule/xmodule/raw_module.py
switch the notification to be a prompt and allow for the user to go to the edit unit page which contains the module in error. Otherwise, present the raw exception message and allow user to go to the course outline page.
handle exceptions inside the outer exception handling
implement PR feedback
get_item -> get_instance
Studio: revises export prompt control copy
change user_track to provide default if event* not in REQUEST
small change in aggregating peer eval grades (rounds up)