Add class `wp-core-ui` to DFW and wpLink as it is required by buttons.css. Props avryl, see #27279

Built from https://develop.svn.wordpress.org/trunk@27858


git-svn-id: http://core.svn.wordpress.org/trunk@27689 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Only show test cookie warnings on submit as caching/proxies may intercept the test cookie for GET requests.

Introduce a new string for when headers are sent and link them to http://codex.wordpress.org/Cookies (new page).

props SergeyBiryukov.
fixes #27373.

Built from https://develop.svn.wordpress.org/trunk@27859


git-svn-id: http://core.svn.wordpress.org/trunk@27690 1a063a9b-81f0-0310-95a4-ce76da25c4cd