When a new user is created in various places throughout the interface,
notifications are sent to the site admin and the new user. Previously, these
notifications were fired through direct calls to `wp_new_user_notification()`,
making it difficult to stop or modify the messages.
This changeset introduces a number of new action hooks in place of direct calls
to `wp_new_user_notification()`, and hooks the new wrapper function
`wp_send_new_user_notifications()` to these hooks.
Props dshanske, thomaswm, boonebgorges.
Fixes#33587.
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