Output fallbacks / theme compat for post format metadata.

* Currently handles link, quote, image, gallery, audio, and video formats.
* `add_theme_support()` for a given post format is now an indicator that the theme handles format-specific metadata.
* If no support for a given format is defined, fallback output will be generated and hooked onto the_content if a post has metadata for that format.
* Fallbacks attempt to be smart about not duplicating data already appearing in the post content itself. Gallery is particularly liberal, looking for any instance of the gallery shortcode in the content, not just an exact match to the gallery shortcode defined in the format-specific meta.
* Compat output defaults to being wrapped in a `div` with a class of `post-format-content`.

Theme authors: please test and evaluate, keeping in mind that the goal is to support user expectations of not losing format-specific data they've entered in the admin when viewing the front-end of their site.

props wonderboymusic, beaulebens, helen. see #23347.


git-svn-id: https://core.svn.wordpress.org/trunk@23450 1a063a9b-81f0-0310-95a4-ce76da25c4cd