Implement demo for forced subtitles

[defagos]

As a user I would like support forced subtitles, if any, to be transparently enabled based on the current audio language.

Remarks

• The behavior must be implemented in video.js directly. It already works with Safari, provided AUTOSELECT and FORCED both have been set to YES for a force subtitles.
• The behavior is implemented in Pillarbox Apple (and requires AUTOSELECT to be set to YES).
• The behavior is implemented in Pillarbox Android (which does not require AUTOSELECT to be set to YES).
• If work cannot be done in video.js directly for some reason we should implement an API that makes it possible to select forced subtitles programmatically, see Handle forced subtitles selection through textTrack #43.

Acceptance criteria

• When playing Apple 16:9 example with the Pillarbox web demo and if subtitles are disabled, forced subtitles are still displayed every 10 seconds or so. This behavior works on all browsers.
• To maintain consistency between browsers, the web implementation must take AUTOSELECT=YES into account for selection on other browsers than Safari.

Tasks

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