Add sorting/filtering by size

[edderiofer]

As per title. I'm imagining it would take the form of "height must be between X and Y" and ditto for width; or there's a way to categorize puzzles by size (e.g. small, medium, large, giant, supergiant; though this probably fails for non-conventionally-sized puzzles).

The two use cases for this that I can see are mostly to complement the existing difficulty filter, and so that one can sort only for mobile-friendly puzzles.

[Robert]

• Explicitly sorting by size is difficult, compare #11.
• Choosing among a set of buckets would be possible, but I agree also that they would end up pretty arbitrary (worse so than difficulty filtering, which also has a somewhat unfortunate interface)
• I think my preferred solution would be to handle this after #14, allowing to query for size in text (e.g. just some predefined query huge, or allowing explicit min-size:100)