Add mgmt cmd to generate anonymized ID mapping

*edit*: to be clearer: this is necessary/useful b/c the graders store user ids as the output of ```unique_id_for_user()```, which makes tracing the data flow for particular users for debugging grading much much harder. Also, instructors often want to reverse-lookup students for other reasons, and we don't have a nice way to support that. Outputting the mapping in CSV is far from ideal, but much better than nothing.

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)