Add Subscription support

Merged Hugo Beauzée-Luyssen requested to merge chouquette/medialibrary:add_collections_mr into master

This MR adds subscriptions support. For now this only means podcasts.

This does not implement local caching and downloading of tracks from collections, it only analyses a podcast rss file as it would a playlist and adds the required datatypes and tables in database to do so.

Edited by Hugo Beauzée-Luyssen

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