Tiled gallery block: don't use full size image, use large image instead

[simison]

Currently, if you upload 5mb image to the gallery, it'll use that 5mb image for the tiny square.

Photonizing URLs will help (#29495) but if the block should work without it, then we should load large image instead.

Here's a PR to Gutenberg Gallery block that uses "large" image as a base instead of full images: WordPress/gutenberg#11682

Additionally, for smaller tiles "medium" size might be enough so there should be some logic to figure out which image to load for the tile. This isn't really needed if we just use Photon.