video_modules now uses edxval to fetch mp4 video values

(moved from https://github.com/edx/edx-platform/pull/4839)
VideoModule now has a new optional field, edx_video_id. If edx_video_id is
specified, use it.

Other notes:
-Django app edxval has been added to the git requirements.
-added edxval as an optional app in cms and lms
-Added edxval to urls