Improve misleading alt lookup query / UI

[danneu]

Right now the UI is misleading:

The recursive query starts with a user_id and then follows all paths to any users that match on ip_address OR tracking cookie.

That means that user1 may be linked to user2 through a sequence of hits like [ip address match -> ip address match -> track match -> ip address match].

The UI is going to show user2 with both IP_ADDRESS and TRACK matches.

However, that's not right since ip address matches are likely false positives, so a path of [ip address match -> track match] should not appear as a TRACK match. I guess it should show up as an IP_ADDRESS match.

I basically want it so that if there's a TRACK match, you know for sure it's a true positive alt.

To pile on this issue a lil more, I want to note that db/hits.js' recursive findAltsFromRequest lookup was originally written when the matches were {ip_address, user_agent, track}. But I have since dropped user_agent, and I realized I needed a user_id entry point so I copy-and-pasted findAltsFromRequest to findAltsFromUserId to quickly edit it so that it starts with a user_id.

This is to say that I need to revisit the query to evaluate whether it can be done in a simpler or better way.