Add `size` to the UI for Gallery Settings in the media modal. Ensure that the TinyMCE view is refreshed when size changes.

Props richardmtl, noplanman, wonderboymusic.
Fixes #18143.

Built from https://develop.svn.wordpress.org/trunk@29779


git-svn-id: http://core.svn.wordpress.org/trunk@29551 1a063a9b-81f0-0310-95a4-ce76da25c4cd