New Post Format UI chooser treatment in response to testing.

* Bigger icons
* Format description under each
* UI hides after choice
* Saving without choosing implies "Standard"
* Helpful text, with "Change format" link

see #24046. props lessbloat, wonderboymusic.

git-svn-id: https://core.svn.wordpress.org/trunk@24006 1a063a9b-81f0-0310-95a4-ce76da25c4cd