Add support for UX reference templates in LMS

I've added simple support for UX reference templates in LMS, in the same way that is already supported in Studio. This is to allow the UX team to create static versions of live pages that they can use for styling purposes, and to share with the wider team before the code is ready.

I've added a first reference template just to be able to demonstrate the approach. I've added a new directory under ```lms/templates``` called ```ux/reference``` where these static files can go. The template I've added can be seen here:

http://localhost:8000/template/ux/reference/instructor_dashboard/membership.html

@cahrens @talbs Please review.