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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Remove debug code from WP_Automatic_Updater::send_email().

Merges [25891] to the 3.7 branch.

props markmcwilliams for initial patch.
fixes #25678.

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Confirm delete_site_option() exists before calling it during an update.

This function won't yet exist when updating from 2.7 [sic].

Merges [25893] to the 3.7 branch.
fixes #25682.

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Always escape URLs at the last possible moment. for the 3.7 branch.

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