Blocks: Add "Export as JSON" action to the `wp_block` post list screen.

Remove the "Quick Edit" for `wp_block`s, too, as they don't need any of that functionality.

Merges [43854] from the 5.0 branch to trunk.

Props noisysocks.
Fixes #45247.

Built from https://develop.svn.wordpress.org/trunk@44220


git-svn-id: https://core.svn.wordpress.org/trunk@44050 1a063a9b-81f0-0310-95a4-ce76da25c4cd