Clean up gallery-mode toggling in post-formats.js and make sure it is set on first load.

see #24062

git-svn-id: http://core.svn.wordpress.org/trunk@24088 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Allow any post format to be default, not just ones with explicit support.

props SergeyBiryukov. fixes #24036.

git-svn-id: http://core.svn.wordpress.org/trunk@24089 1a063a9b-81f0-0310-95a4-ce76da25c4cd