[bbc] Improve iPlayer extraction for audio description

[dirkf]

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In issue #22986, the reporter noted that the audio-described version of an iPlayer programme was not retrieved when using the AD URL. This seems wrong.

The extractor used playlist data from http://www.bbc.co.uk/programmes/PID/playlist.json, but this doesn't include the AD version. @Vangelis66 (thanks) pointed out some places where playlist data including the AD version might be found.

On investigation it seems that the new React/Redux version of iPlayer sends JS that sets the application state from JSON, and this JSON includes all the data that is needed for iPlayer extraction.

This technique might render the tactics previously implemented in the extractor obsolete, but I left them in as fall-backs.

Instead of fetching the iPlayer page from the given URL, it would be possible to get similar data from IBL (NB slightly different keys, eg REDUX episode vs IBL episodes). However, the other (possibly obsolete) extraction tactics require the page, so at the moment the REDUX data is available with no further requests.

Resolves #22986.