Nonce refresh:
- Update the heartbeat nonce when refreshing nonces on the Edit Post screen.
- After a user logs in from the auth-check dialog, speed up heatrbeat to check/refresh nonces on the Edit Post screen.
- Speeding up heartbeat: bring back the setting how long it should last (how many ticks).
- Add 'heartbeat-nonces-expired' jQuery event when nonces have expired and the user is logged in.
See #23295, see #23216.

git-svn-id: http://core.svn.wordpress.org/trunk@24528 1a063a9b-81f0-0310-95a4-ce76da25c4cd