Roll out email_on_added_notifications

What does this MR do and why?

Roll out email_on_added_notifications. See:

This is already enabled site-wide for GitLab.com; this change means all instances will send email notifications when a note is edited to add a mention of a previously unmentioned user.

How to set up and validate locally

Testing the change itself is per !224837 (merged)'s validation instructions, except the FF enablement is no longer required.

MR acceptance checklist

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