FEATURE: image grid in posts (experimental) (#21513)

Adds a new `[grid]` tag that can arrange images (or other media) into a grid in posts. 

The grid defaults to a 3-column with a few exceptions:

- if there are only 2 or 4 items, it defaults to a 2-column grid (because it generally looks better)
- on mobile, it defaults to a 2-column grid
- if there is only one item, the grid has no effect

lib/svg_sprite.rb

@@ -203,6 +203,7 @@ module SvgSprite
         tag
         tags
         tasks
+        th
         thermometer-three-quarters
         thumbs-down
         thumbs-up