Add "sort by" features to the search

[edderiofer]

Add options to sort by puzzle size, by # of faves, by # of solves, etc.

[robx]

I see how these would be useful, but am unsure I want to add them for a couple of reasons:

1. robust result paging is tricky, and the solution that I've implemented for current timestamp-based ordering uses a dedicated key; generalizing this to different sort orders would be at least a bit of work, and I expect it would run into some more serious difficulties I'm not aware of at this moment
2. it's much harder to remove the feature again once it's been out there, if it turns out to be too expensive; particularly, this is a reason not to offer a naive implementation of ordering on such fields at this point