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

Merges [25882] to the 3.7 branch.

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.

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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.

Merges [25883] to the 3.7 branch.
see #25603.

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About page: Cache our zxcvbn calls and limit the variation of the animation.

Merges [25884] to the 3.7 branch.

Diff is mostly whitespace.

props jorbin, azaozz.
fixes #25603.

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3.7-RC2-25888

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Avoid notice with search feeds.

Merges [25889] to the 3.7 branch.

fixes #25677.

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