Hide sparse attributes

[ptrwtts]

We added a bunch of collections and attributes, and it's very possible that some of them are inefficient. For example, on the veefriends collection, I noticed some attributes that are different for every token. We should basically have a threshold for the number of unique attribute values, and over that, we give the attribute a rank of -1.

Ideally, I could do this in the metadata script, but I didn't think about it until after! Is it something we could easily do with a SQL query?