Add Mailer for Namespace Storage Limit Exceeded

What does this MR do and why?

Add a mailer for when the namespace storage limit is exceeded.

This MR only adds the mailer. Code to send the emails does not exist yet.

This mailer and the messages are based on the CiMinutesUsageMailer and its views.

The exact text of the messages may ultimately need adjusted.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/349682

Screenshots

Storage warning email

HTML

Plain text

Storage exceeded email

HTML

Plain text

How to set up and validate locally

1. Navigate to http://localhost:3000/rails/mailers
2. Click on the items below Emails/Namespace Storage Usage Mailer, limit_warning and out_of_storage
3. Click the links in the email messages to verify that they go to the right locations

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.