Twenty Fifteen: prevent too-wide tables from popping out of posts.

Props iamtakashi, fixes #30420


Built from https://develop.svn.wordpress.org/trunk@30566


git-svn-id: http://core.svn.wordpress.org/trunk@30555 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: remove unneeded custom header admin callbacks.

Props philiparthurmoore, fixes #30432.


Built from https://develop.svn.wordpress.org/trunk@30567


git-svn-id: http://core.svn.wordpress.org/trunk@30556 1a063a9b-81f0-0310-95a4-ce76da25c4cd