add sysadmin dashboard for Studio

It is useful for sysadmin staff to be able to:

* Purge the mongodb of course fragments, which match course id's but are not deleted through the normal django management command (which deletes only entries linked in the course tree).  We've seen such course content corruption in the database happen several times, associated with course teams importing `.tar.gz` files with errors.

* Load course from a git url via a web interface

* Get JSON dump of course content directly from the mongodb (eg for quick and exact transfer of course modules to another Studio instance).

This PR provides those features, via a rudimentary but functional dashboard.

No tests provided.