Create a mailer to be called when users exceed seats_in_use
Problem to solve
As a part of the issue https://gitlab.com/gitlab-org/gitlab/-/issues/348487, an email has to be sent to the admin of the group when the count of users exceed seats_in_use.
Proposal
We need to create a mailer which will be called in background. The mailer would accept the namespace_id and would send the following email
Subject line: Additional charges for your GitLab subscription
Dear [customer],
You have exceeded the number of seats in your GitLab subscription A-3834954834 by [seats owed number]. Even if you've exceeded the seats in your subscription, you can continue to add users, and GitLab will bill you a prorated amount for any seat overages on a quarterly basis.
You can find more information about the quarterly reconciliation process in our documentation.
Please reach out if you have questions, and we’ll be happy to assist.
Thank you for your business!
GitLab Billing Team
Related to https://gitlab.com/gitlab-org/gitlab/-/issues/348487