Update audio and video file field formatters to include track elements

Problem/Motivation

Part of the audio/video accessibility plan - 🌱 Provide authors with tools to manage transcripts and captions/subtitles for local video and audio Active

Prerequisite: 📌 Provide a FieldType and FieldWidget to represent a WebVTT file for the HTML track element Needs work . That issue provides the data model and authoring UI to upload WebVTT track files to accompany audio/video elements.

This issue is about rendering the track elements in the HTML output of fields.

Proposed resolution

• Add a new setting to the video field formatter. Site builders can specify a field containing tracks relating to this video field.
• Pass the track file information to the video field template. Support all the attributes of the <track> element.
• Render the track elements inside the video element.
• I expect the markup changes can qualify for Stable and Classy backwards compatibility policy, because:
  • We're addressing a WCAG level-A failure, and
  • Existing sites which don't have the caption field yet, the HTML track element just doesn't need to be rendered at all.
• Provide the same functionality to the audio file field formatter. Much code can be shared in FileMediaFormatterBase.

Remaining tasks

• Improve the video formatter setting which species the track field. Show a select for qualifying fields (i.e Media Track fields) so users can't choose the wrong type of field, and avoid mistakes in typing the maching name.
• Add the same functionality to the audio file field formatter
• Update config schema for media file formatters?
• Tests for new functionality.
• ... anything else?

User interface changes

Adds a new setting to the video field formatter, so site builders can specify whether to include track elements from another field.

API changes

None.

Data model changes

Adds a configuration option to the video field formatter.