See #170: subscribe to 3d-party RSS feeds in Funkwhale

See #170 (closed):

• Channel has a random actor
• Library is private
• Disable RSS feed for external channels
• Subscribe by RSS feed on existing Funkwhale channel
• We use the channel UUID in the url (since actor is random, and we don't expose the actor at all in the API results)
• Channel is unreachable if not subscribed to
• Subscribing checks if the channel has valid XML / entries
• Hide channel actors attributed to service_actor
• Hide channel actor from channel response if it's service actor
• Include alternate link to activity+json actor, to avoid having duplicate actors / rss feeds
• Tasks and settings to refetch feeds periodically
• Update denormalization for created uploads
• Honor domain block when fetching RSS feed
• Search bar/page to subscribe to feed
• Test with a Funkwhale emitted feed

cf https://mastodon.eliotberriot.com/@eliotberriot/103815747575835838