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.

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git-svn-id: http://core.svn.wordpress.org/trunk@25794 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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.

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

Diff is mostly whitespace.

props jorbin, azaozz.
fixes #25603.

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Avoid notice with search feeds.

fixes #25677.

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Remove debug code from WP_Automatic_Updater::send_email().

props markmcwilliams for initial patch.
see #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].

fixes #25682 for trunk.

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Always escape URLs at the last possible moment.

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