Add mailer with templates

What does this MR do and why?

Refs: #367549+

Describe in detail what your merge request does and why.

Screenshots or screen recordings

Current screenshot:

Old screenshots:

How to set up and validate locally

1. Checkout sf/feature/367549-email-over-limit-users
2. Start GDK
3. Visit http://localhost:3000/rails/mailers/notify/reached_free_user_limit_email.html?locale=en

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.