Post Format UI.

* Icons
* Selection
* Prompt text
* Special fields
* Styling
* Sparkles

This is going to need testing, polish, and love.

see #19570. props melchoyce, helen, wonderboymusic, lessbloat, rachelbaker, aaroncampbell, DrewAPicture, ryelle.

git-svn-id: https://core.svn.wordpress.org/trunk@23843 1a063a9b-81f0-0310-95a4-ce76da25c4cd