Remove old control/accordion section title gradients that were still appearing in the customizer and reinstate a divider between the customizer sidebar and the preview. props ryelle, helen. see #25858.

Built from https://develop.svn.wordpress.org/trunk@26165


git-svn-id: http://core.svn.wordpress.org/trunk@26076 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix JSHint errors in postbox.js.

props atimmer.
fixes #26006.

Built from https://develop.svn.wordpress.org/trunk@26167


git-svn-id: http://core.svn.wordpress.org/trunk@26077 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Heartbeat: clean up code style, better naming, better code structure. Props carldanley, props evansolomon.
Changes:
- Add connectNow() public method to trigger a connection immediately.
- Remove the "skipping" when no data to send.
- Change the default interval to 60 sec.
- Fix resetting the next connection time when changing the interval.
See #25073.
Built from https://develop.svn.wordpress.org/trunk@26169


git-svn-id: http://core.svn.wordpress.org/trunk@26078 1a063a9b-81f0-0310-95a4-ce76da25c4cd