Add previews for missing access token emails

What does this MR do and why?

Add previews for 2 missing access token email templates, access_token_created_email and access_token_revoked_email.

Screenshots or screen recordings

How to set up and validate locally

1. Restart gdk with gdk restart
2. Visit rails/mailers http://gdk.test:3000/rails/mailers
3. Confirm two templates appear:
   • access_token_created_email: http://gdk.test:3000/rails/mailers/notify/access_token_created_email.html?locale=en
   • access_token_revoked_email: http://gdk.test:3000/rails/mailers/notify/access_token_revoked_email.html?locale=en

More information at: https://docs.gitlab.com/ee/development/emails.html#mailer-previews

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.