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
Penar Musaraj