Added REST API for missing mobile videos

@nasthagiri For https://openedx.atlassian.net/browse/MA-79

So if I'm reading this right, it's using the fact that file size information is only ever stored in VAL, so a VideoModule summary that has a file size of zero means that there exists no VAL entry for this item in the requested encoding profile (mobile_low, in this case).

This seems pretty fragile and unintuitive (esp. since this method lacks a docstring explaining this). We have the list of edx_video_ids that VAL is aware of for this course -- it shouldn't be that much more code to make checking against that more explicit, and to be able to give more precise information: e.g. "edx_video_id no longer exists in VAL" vs. "edx_video_id exists but there's no entry for this encoding", etc.