Mopidy-youtube

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- preload support for mopidy-tubeify

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- changes

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- youtube channel support
- tidy up autoplay
- fix some thumbnail stuff
- deprecate scrAPI and bs4API in favour of jAPI
- try to reduce trips to end points
- add caching (and related web client)

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- revised backend and other fixes

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- release to fix breaking changes and other minor bugs

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- first python3 revision