Add filtering by variant type

[Clozent]

It would be nice to be able to filter puzzles not only by their type, but by their variant types as well.
For example, I like sudoku, and I want to find some puzzles of a few specific sudoku variants I like, such as kropki sudoku.
With a variant filter, all I would need to do is to filter by the sudoku puzzle type, and then also add the "kropki" variant to the filter.

I personally think this would work the best if implemented with #14, since that would allow for "logic operations" when filtering, such as a looking for a puzzle that features two or more variant types at once, or excluding certain variant types.