Implement indexer adding new /search/ multiboard feed for search results

[tomcasaburi]

The UI should be similar to 4chan's /search/: https://find.4chan.org/.

I'll run my own plebbit-indexer instance for this. Later, we can add a setting to manage a list of indexer providers, which should be handpicked by 5chan devs to ensure they only index 5chan boards.

It should only index 5chan default boards for now. Later on, it could also index boards applying to be a 5chan directory, to increase their discoverability, perhaps with a filter to only show boards with x minimum upvotes in each directory pubsub voting page.