Simple frontend for students and teachers. Add calculation of active series. I still need to add link from /

Summary:
Route with stub files for coach and students dashboard

more consistent file naming with some display adjustment

change route name and fix file names

Add counting active students and teachers

Drop mongo collection since we are recomputing always whole range

Add more comments and drawing of active series of teacher and students

Test Plan: /coaches renders a page

Reviewers: jace, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2427