Attempt to use YouTube RSS feeds where possible to index new media for existing sources

[meeb]

Once the first index is complete for a source it should be possible to use RSS feeds to scan for new media as discussed in #48 - this should lower the number of requests may trigger rate limiting. This needs to be an additional feature to the current indexing method and fall back to the current indexing method if no "known" video ID is present in the RSS entries (determining that there may be more new results in the RSS feed and the indexer may miss content). These RSS feeds are only appropriate for YouTube sources so need to be developed in a way that doesn't impact the future roadmap where non-YouTube sources will be supported.

Example feed formats:

https://www.youtube.com/feeds/videos.xml?playlist_id=some-id
https://www.youtube.com/feeds/videos.xml?channel_id=some-id

A metadata request with youtube-dl is likely still required to obtain the internal channel_id and playlist_id from a user-supplied URL of https://www.youtube.com/channel or https://www.youtube.com/c/channel etc.

This will be blocked by #33 which will have to be completed first.