RSS feeds, continued

[BasiqueEvangelist]

mineblock11 seems to have stopped working on #1159. This PR is a continuation of that PR, with some additional changes.

• [x] User notification feed removed (for now) Just passing the token via a header feels like it wouldn't be implemented by many people, and putting the GitHub token in the path seems a bit worrying, especially if the URL gets leaked. Having a separate feed secret would be nicer, but that requires changes on labrinth.
• [x] Much more metadata is put in the feed The version items have their loaders and game versions as categories and the publisher is listed as the author, the project's team is listed in the feed, et cetera.
• [x] Project versions page links to feed The most basic form of this is to link to them via <link> elements, but showing off the feed visually would also be pretty nice.

[BasiqueEvangelist]

Ready for review, just in case modrinth/labrinth#527 isn't accepted

[Prospector]

would it be possible for you to fix merge conflicts