Add a course list extension that shows all courses an instructor can manage

[jhamrick]

Fixes #1060 and related to #1030

This creates a new extension called course_list that displays all courses available to an instructor, including local ones (that are loaded into the same single user notebook) and JupyterHub ones (which are found by querying the JupyterHub services). Currently it looks something like this:

• add docs
• add tests
• tes with a full JupyterHub setup

@damianavila @BertR @sigurdurb

[jhamrick]

There are minimal docs at least for how to install/disable the extension, so I will go ahead and merge and once #1114 is merge I will add a little more in a separate PR.