Add a list view for metadata editor.

This is to be used with VideoAlpha's functionality allowing a choice
of multiple HTML5 video sources.
Convert Video Alpha to metadata-only.
Unstyled prototype for new list editor design.
Studio: revises visual layout/presentation of multi-value metadata fields
Studio: corrects minor styling/formatting for multi-value metadata fields
PR comment cleanup.
Merge pull request #455 from edx/peter-fogg/remove-videoalpha-xml

Peter fogg/remove videoalpha xml