Simplify and reduce the new media/content extraction functions.

The URL extraction function is now get_url_in_content(). For more, see #24202.

Also adds filters to get_post_galleries() and get_post_gallery(). fixes #24309.



git-svn-id: http://core.svn.wordpress.org/trunk@24682 1a063a9b-81f0-0310-95a4-ce76da25c4cd