Merge "video sources" and "download video" into the same field

Ticket: [BLD-364](https://edx-wiki.atlassian.net/browse/BLD-364).

Change the download video field to a dropdown that will allow students to download the first source listed in the alternate sources.

Alternate sources = AS, Download video = DV.
a) If DV exists and DV != AS: show DV field on front-end as not-editable and clearable. Dropdown is a new field `download_video` and it has value `True`. Student downloads DV. If teacher want to change DV, he should clear DV field and put video in AS.
b) If DV is cleared it is not shown anymore.
c) if DV exists and DV in AS, do not show DV field, dropdown has value `True`.

If the DV is the same as one of the AS videos, the DV link shouldn't be used. The AS should be used instead, so that we do not force authors to delete material in order to revise unless we absolutely must.

@nedbat , @valera-rozuvan , @auraz, @frrrances please review.