Send a notification when a new email is added

What does this MR do and why?

Implements #337635 (closed). GitLab now sends an email to the primary address each time a user adds a new email address to the profile.

Screenshots or screen recordings

Courtesy of @dmoraBerlin

How to set up and validate locally

To see a preview of the email template, go to /rails/mailers

To test the email notification:

• Go to your profile;
• Add an email address;
• Visit /rails/letter_opener

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.