Twenty Fifteen: correct escaping to render HTML in no-content admin message.

Props dmchale, fixes #30011.

Built from https://develop.svn.wordpress.org/trunk@29947


git-svn-id: http://core.svn.wordpress.org/trunk@29696 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Introduce `customize_preview_$setting->type` action to handle multiple settings of the same type.

props celloexpressions.
fixes #29165.
Built from https://develop.svn.wordpress.org/trunk@29948


git-svn-id: http://core.svn.wordpress.org/trunk@29697 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: centre gallery thumbnails

Props iamtakashi, fixes #30019


Built from https://develop.svn.wordpress.org/trunk@29949


git-svn-id: http://core.svn.wordpress.org/trunk@29698 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Extract content markup for panels to its own method, `WP_Customize_Panel::render_content()`.

This allows to override the behavior of a panel, or even to completely replace its contents with something other than controls or sections.

props celloexpressions.
fixes #29324.
Built from https://develop.svn.wordpress.org/trunk@29950


git-svn-id: http://core.svn.wordpress.org/trunk@29699 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: Swap order of description and caption so the caption is underneath an image on image attachment pages.

Props iamtakashi, fixes #30020.


Built from https://develop.svn.wordpress.org/trunk@29951


git-svn-id: http://core.svn.wordpress.org/trunk@29700 1a063a9b-81f0-0310-95a4-ce76da25c4cd