Send a notification when a new access token is created

What does this MR do and why?

Implements #337591 (closed). Each time the user creates a new personal access token, GitLab sends a notification email to the user.

Screenshots or screen recordings

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How to set up and validate locally

To test the email notification:

• Go to your profile;
• Add a new personal access token;
• Visit /rails/letter_opener

MR acceptance checklist

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