Post Formats: use the content body for the body of the Quote post format.

* Searchable, better editing tools
* Less cluttered Post Format UI

props kovshenin, DrewAPicture. see #24009

git-svn-id: https://core.svn.wordpress.org/trunk@24034 1a063a9b-81f0-0310-95a4-ce76da25c4cd