Administration: Prevent repeat emails for identical plugin or theme auto-update attempt failures.

This change adds a throttle mechanism to plugin and theme auto-update failure emails using similar logic to the email sent for a Core auto-update.

The first time a plugin or theme auto-update fails, the package and `new_version` will be tracked in the `auto_plugin_theme_update_emails` option. An email for this specific update attempt will not be resent.

However, if this update fails again and non-repeat failures or successful updates are also present, then the failure information will be included in that email (an email needs to be sent for the new events regardless).

Props johnbillion, arpitgshah, desrosj, audrasjb, pbiron, earnjam.
Fixes #50448.
Built from https://develop.svn.wordpress.org/trunk@48397


git-svn-id: https://core.svn.wordpress.org/trunk@48166 1a063a9b-81f0-0310-95a4-ce76da25c4cd