Add Mailgun suppression bypass header to reset password mail

What does this MR do and why?

Users whose email address has been marked as spam by Mailgun and who loose their password, are effectively locked out of their account. This MR adds a Mailgun suppression bypass header to the reset password instructions email so that those users are at least able to regain access to their account.

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/188

How to set up and validate locally

1. While logged out, visit http://localhost:3000/users/password/new
2. Fill in an email address for an existing user and click Reset password
3. Verify in the logs that this header is added X-Mailgun-Suppressions-Bypass: true

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.