Investigate CSS grid for puzzle list page

[Drew Fisher]

To date, the puzzle list page has been using flexbox with either fixed pixel-count or percentage column widths, which behaves predictably but has had some issues with avoiding wrapping with certain font sizes, and it also sometimes means we're not making particularly good use of the horizontal space, given particular puzzle titles, tag lists, or answers.

It might be worth seeing if CSS grid can do a better job of allocating the space based on the current set of puzzle titles/answers/tags.

One thing we might have to think carefully about is whether we want to have all the puzzles be part of a single grid, or if each related puzzle group should do the locally-best layout. I can imagine arguments in either direction -- uniformity looks pretty, but the eye might be able to notice the different groups a bit more if they have some variation. Worth taking a look at and seeing if we can make anything we like better.