Started removing XML from video editor.

TODO: This breaks the 1.5x and .75x speeds. I'm still looking into
why.

TODO: VideoDescriptor inherits from RawDescriptor in order to
use the from_xml and export_to_xml methods. This seems really ugly,
though; I'd rather find a better way to do this.
Fixed .75 and 1.5 speeds playing at 1.0 speed.
Changed description of source and track fields.
Fix failing acceptance tests and allow videos to be imported from XML.
Enable exporting video to XML for backwards compatibility.
Merge peter-fogg/fix-video-captions-setting.
Update acceptance tests for captions to use metadata settings editor.
Convert Floats to StringyFloats.
Remove data attribute from templates/video/default.yaml.
Prevent showing video speeds that aren't set by course author.
Convert show_captions to StringyBoolean.
Update and fix acceptance to show captions when correct.
Fix failing test bug.
Set Youtube ID defaults to the empty string.
Update display names and help text for video metadata.
Fix times not being parse from HH:MM:SS format into floats and update video module logic tests.
Update failing Jasmine tests for video.
Remove outdated unit test for VideoModule constructor.
Update XML in old video tests. This change tags like <video youtube="1.0:abcdefg"> to <video youtube_id_1_0="abcdefg".
Update XML in old video tests. This change tags like <video youtube="1.0:abcdefg"> to <video youtube_id_1_0="abcdefg".