Allow to switch get_search_form() to HTML5 with an add_theme_support() call. fixes #23850.

git-svn-id: https://core.svn.wordpress.org/trunk@24225 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve the UX and flow of the Image post format.

* Drag-and-drop
* Textarea for HTML/URL hidden to start
* Bigger click target for entering Media modal
* Image preview height-contrained to 300 pixels
* Hid the alignment, size, etc options that didn't have an effect anyway
* Got wp-format-{format} into the <body> class

props DrewAPicture, aaroncampbell, markjaquith, azaozz. see #24291.

git-svn-id: https://core.svn.wordpress.org/trunk@24226 1a063a9b-81f0-0310-95a4-ce76da25c4cd