Create email message encouraging group creator to invite their teammates

What does this MR do and why?

This MR adds the email message that will be sent as part of this experiment. It will be sent to the user 20 minutes after creating a top-level group to encourage them to invite their teammates.

Note: This MR only adds the email message. The actual delivery will be implemented in !72470 (merged).

Screenshots or screen recordings

How to set up and validate locally

1. Open Rails console

   # in your terminal inside /gitlab-development-kit/gitlab
   bin/rails c

2. Send the email

   # inside Rails console
   # you can use any user or group
   Notify.in_product_marketing_email(user.id, group.id, :invite_team, 0).deliver_now

3. Open letter_opener and verify the content of the email

   🆘 If you're having trouble viewing the email in letter opener (CSP problem?) it might be because of the issue that !72830 (merged) is trying to fix. Don't hesitate to let me know if you need help testing!

   It should look like this 👇🏼

   

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.