Make SPOC gradebook an API feature

rather than just a hacked in feature. Also point tests from the legacy implementation
at the new dash API implementation.

@adampalay - feel free to nominate a second reviewer :)

Background: The original gradebook function was defined in `lms/djangoapps/instructor/views/legacy.py`. In #2865 I moved this function into `lms/djangoapps/instructor/views/instructor_dashboard.py` which was incorrect; I ought to have moved it into `views/api.py`. This PR removes this function from both `legacy.py` and `instructor_dashboard.py,` just having 1 definition in 1 file and all references to this function point at this one implementation. It also moves the URL pattern into the api urls file, and out of `lms/urls.py`.