Introduce a send_core_update_notification_email filter to control whether to send update notifications.

Currently, this is dependent on whether the automatic updater is enabled (generally). This is likely to be decoupled in a future release.

This refocuses the auto_core_update_send_email filter to specifically be for success/fail emails for auto updates, while the new filter is more general.

see #25603.


git-svn-id: https://develop.svn.wordpress.org/trunk@25882 602fd350-edb4-49c9-b593-d223f7449a82
About page, updates section: Don't show a message on about.php if the updater is 100% disabled. Don't show it if the user can't update core. Don't show the email if notifications are disabled. New string.

see #25603.


git-svn-id: https://develop.svn.wordpress.org/trunk@25883 602fd350-edb4-49c9-b593-d223f7449a82
About page: Cache our zxcvbn calls and limit the variation of the animation.

Diff is mostly whitespace.

props jorbin, azaozz.
fixes #25603.


git-svn-id: https://develop.svn.wordpress.org/trunk@25884 602fd350-edb4-49c9-b593-d223f7449a82