Add sysadmin dashboard

This PR provides an integrated dashboard for a sysadmin to be able to:

- get a summary of how many users are on the system
- get summary information about all courses loaded into the modulestore
- get a list of enrollment (and number of staff) in each course for all courses
- load courses from git / github
- delete courses
- view the transcript of errors produced when importing a course (git logs)

Included is a script which can be used to load course XML content from github.  Note that the git import logging and log viewing requires mongoengine (http://mongoengine.org/).

This functionality is enabled by the new `MITX_FEATURES["ENABLE_SYSADMIN_DASHBOARD"]` flag.

This code is in use in the production residential MITx systems at MIT.