Allow a 'expand' filter that expands the matched set out by a BFS

[jkomoros]

A use case: "search for cards that match a given query, and then also include any cards that are within 1 inbound or outbound link"

A few ways of doing it:

1) Having a expand/<others> that is a special kind of filter that runs after the rest of the logic runs and is given the currently matched set and then can expand it. This requires all the collectiondescription logic to be updated and seems clunky. 2) Make it so every filter in a chain is passed the results of the previous filters in the chain... but then how do you EXPAND it? 3) (The right answer, likely) is introduce an expand query that takes two sub-filters. It's basically a union filter, but where it does the left side of it, then passes that set to the second part, and then returns the full union

[jkomoros]

Also don't forget to update the filter creator UI

[jkomoros]

• [x] CardBFS takes an array of cardIDs as the starter set
• [x] Factor out the cardLinksConfigurableFilter to have the main machinery be factored out away from filter parsing
• [x] Make it so an override card set can be passed to cardBFSMaker
• [x] Add a URL_PART_LINK_FILTER
• [x] Actually create the expand filter
• [x] Make it so piecesForConfigurableFilter doesn't just consume all remaining pieces of the filter (that also wouldn't work for 'combine', either...). Right now if you do an expand, the little text box for the first sub-filter shows all-cards/children/+key-card-id, instead of just all-cards.
• [x] Make the expand filter actually do its thing
• [x] Make the expand filter actually do an expansion based on the second filter
• [ ] Add a similar-to filter that shows cards that are similar to, above some threshold (hmmm, you can't use LIMIT for that because limit is a special filter...)

[jkomoros]

We can just skip the cardID in the second part of filters, but the parsing machinery is actually hard to change, so it's easiest to just have there still be a (noop) card ID there. The KEY_CARD_PLACEHOLDER_ID is fine, and that's the default anyway so 🤷

[jkomoros]

All of the remaining parts are now captured in other issues

[jkomoros]

7ffb5d3d4f092534bf2e85be51b06eecb1d22298 is also part of this