Adds mailer for Block Seat Overages
What does this MR do and why?
When blocking seat overages (wip), namespaces will be made read-only if they go over their seats in the subscription.
This MR adds an email that will be sent to namespace owners notifying them that this has happened.
The mailer will be used in a follow-up MR (wip: !150286 (closed)) and will be triggered whenever the namespace goes over their seat limit.
This approach follows the same format as the FreeUserCap mailer.
Refs https://gitlab.com/gitlab-org/gitlab/-/issues/455746
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