`WP_REST_Block_Renderer_Controller` allows rendering of server-side rendered blocks, whilst `WP_REST_Blocks_Controller` allows retrieving of reusable blocks.
Merges [43805] and [43806] from the 5.0 branch to trunk.
Props desrosj, danielbachhuber, pento, Presskopp, swissspidy.
See #45065, #45098.
Built from https://develop.svn.wordpress.org/trunk@44150
git-svn-id: http://core.svn.wordpress.org/trunk@43980 1a063a9b-81f0-0310-95a4-ce76da25c4cd
