Use the `/indexes` api from @singingwolfboy and my work. Let's not have many overlapping apis (simplify, reduce, etc ...) Note, `indexes` is intended to be more general than just courses which you can register for but also include any name-addressable container of xblocks w/ optional branches; so, we may need to filter to just courses or just active courses.

e.g., `GET /edx-api/courseware/v1/indexes/active`