Block Editor: Backport inclusion of new editor settings to the core.

This commit includes in the core settings for the gradients theme API stabilized in https://github.com/WordPress/gutenberg/pull/20107, and for the image sizes required for the latest posts feature image https://github.com/WordPress/gutenberg/pull/17151/.

Props: youknowriad, ryelle.
Fixes #49389.
Built from https://develop.svn.wordpress.org/trunk@47240


git-svn-id: https://core.svn.wordpress.org/trunk@47040 1a063a9b-81f0-0310-95a4-ce76da25c4cd