Updated playlist management API

This is a cleanup of the /playlists API endpoints, in particular:

• removal of the /playlist-tracks endpoints
• moved all the removed /playlist-tracks actions such as moving a track in the playlist or deleting it to playlist/{id}/move and playlist/{id}/remove
• Don't expose playlist tracks ids anymore. We identify them by their index in the playlist

This makes the API more consistent and easier to deal with and document.

cc @Sporiff

changed milestone to %1.0

added AreaBackend AreaFrontend ComponentPlaylists labels

added 1 commit

• 6673731c - Removed unused tests

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added 1 commit

• a0c91d81 - Fix typo

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added 3 commits

• a0c91d81...a1865cf9 - 2 commits from branch develop
• 909e3c4f - Merge branch 'develop' into 'playlist-tracks'

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enabled an automatic merge when the pipeline for 909e3c4f succeeds

merged

mentioned in commit c42257b9