support freetube playlists import/export

[ghost]

Closes #3816

[x] tested import FreeTube playlists to app
[x] tested export Piped / LibreTube playlists from app
[x] tested export FreeTube playlists from app

[Bnyro]

Maybe we can rename the string of the Piped option to "Piped / LibreTube"? Otherwise users that exported it earlier probably don't know which format they exported it to.

[ghost]

Maybe we can rename the string of the Piped option to "Piped / LibreTube"? Otherwise users that exported it earlier probably don't know which format they exported it to.

done

[Bnyro]

Only some minor things from my side, looks good to me otherwise!

[Bnyro]

Have you tested whether it works well after the latest changes?

[ghost]

Have you tested whether it works well after the latest changes?

Yes and kind of. I don't have Piped account or Youtube account. So my playlists come from FreeTube export. And then once imported the FreeTube playlists in the app, I exported it to Piped & FreeTube formats.

It would be great if someone has either an Piped account or Youtube account to help testing by exporting playlists, then importing into the app to ensure the most current formats of these 2 services are consistent to FreeTube's export formats.

[x] tested import FreeTube playlists to app
[x] tested export Piped / LibreTube playlists from app
[x] tested export FreeTube playlists from app

[Bnyro]

Tested the Piped format and the YouTube CSV and works as expected. Thanks for your PR!