added POST/GET for video models

Initial part of the upload portion of the API. Video Models can
be uploaded via POST as a list of dictionaries. Matching entries
will be updated and new entries will be created. Errors are
returned in a list of tuples with the message and input. Single
entries can be returned by edx_video_id via GET.

Other notes:
-Django Rest Framework upgraded for APIView 2.3.5->2.3.14
-Field "client_title" -> "client_video_id" in the Video Model

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