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.
Rename variables for compatibility with original video player.
Ensure backwards compatibility with VideoModule's XML format.
Clean up VideoAlpha test hack.
Add VideoAlpha acceptance tests.
Unstyled prototype for new list editor design.
Add list view tests and update Youtube display names.
Cleanup in response to PR comments.

Move video alpha acceptance tests into the same file as video; fix
some tests; correct non-editable metadata mutating the superclass's
data; fix typos; rename test data to make it more clear that it's just
dummy data.
Studio: revises visual layout/presentation of multi-value metadata fields
Studio: corrects minor styling/formatting for multi-value metadata fields
Fix up list view JS tests.
Disable "add setting" button if there's already a blank one.
Clean up some duplicate code.
Changes per PR comments.

Add test for List type; add i18n; clarify html5_sources help text;
re-enable 'add settings' button on keypress.
Correctly enabled videoalpha captions.