Check if WP_DEFAULT_THEME starts with twenty before asserting it is in default theme array

This fixes an issue that if you change WP_DEFAULT_THEME and run core unit tests, the tests fail since your theme isn't one of the hard coded lists of default themes. We need to keep this test to make sure that we update the array of default themes for use in other tests.

If we ever change the naming convention for default themes, this will need to be updated.

props nacin for initial idea
fixes #29925



git-svn-id: https://develop.svn.wordpress.org/trunk@29946 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: correct escaping to render HTML in no-content admin message.

Props dmchale, fixes #30011.


git-svn-id: https://develop.svn.wordpress.org/trunk@29947 602fd350-edb4-49c9-b593-d223f7449a82
Customizer: Introduce `customize_preview_$setting->type` action to handle multiple settings of the same type.

props celloexpressions.
fixes #29165.

git-svn-id: https://develop.svn.wordpress.org/trunk@29948 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: centre gallery thumbnails

Props iamtakashi, fixes #30019



git-svn-id: https://develop.svn.wordpress.org/trunk@29949 602fd350-edb4-49c9-b593-d223f7449a82
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.

git-svn-id: https://develop.svn.wordpress.org/trunk@29950 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: Swap order of description and caption so the caption is underneath an image on image attachment pages.

Props iamtakashi, fixes #30020.



git-svn-id: https://develop.svn.wordpress.org/trunk@29951 602fd350-edb4-49c9-b593-d223f7449a82
Fix formatting in [29946].

props TobiasBg.
fixes #29925.

git-svn-id: https://develop.svn.wordpress.org/trunk@29952 602fd350-edb4-49c9-b593-d223f7449a82