Edit User

API now provides the ability to edit User data.  Currently only the 'is_active' field can be edited, and we can add more support as the need arises.  Access this behavior via POST /users/(user_id)

In addition, the 'is_active' value can be provided during initial creation of the user, as part of the POST /users payload.  If no value is provided the system default (True) is used.

I removed the DELETE /users/(user_id) operation which we were previously using to set 'is_active' to False.  This was an initial guess on my part at the beginning of the project, but it does not align semantically with how we are using the is_active field in edX.  Since we have no real use case for deleting a user, it's gone until the time comes when we want it back.

Lastly, when I removed the DELETE operation, the unit tests exposed an issue with DELETE /sessions/(session_id) -- I added a call to logout() which appears to have resolved the issue.

@chrisndodge @martynjames @ziafazal @muhhshoaib, FYI