Media captions and alternates

[eloyer]

Adds the ability to integrate WebVTT caption tracks with HTML5 video via two methods:

• Adding one or more dcterms:relation metadata properties to the video media item, each containing a path to a WebVTT file
• To specify the title and language of each file, JSON can be included in the dcterms:relation property instead, in the following format:

{
  "src": "captions.vtt"  // path to WebVTT file
  "label": "English",    // display title of the track
  "kind": "captions",    // can also be "subtitles"
  "srclang": "en-us",    // language code
  "default": true        // use this track as the default
}

Also adds the ability to add "alternates" to any Scalar media item, by adding one or more dcterms:relation properties, each of which contains the Scalar URL slug for the item to be related (media or page). The titles and descriptions of these items will be displayed in a "Related" tab wherever the media is embedded.

[eloyer]

@craigdietrich Would be great to get your eyes on this when you have a chance!