[SaaS, Introductory email] - send email to all group owners

Description

As part of &7229 (closed), we will be sending few mails 1 day after a subscription has been purchased or renewed.

In #4000 (closed) we went with de-scoped list of users to send to but that doesn't solve the problem

We should send an email to all group Owners (not just the group Owner who purchased the subscription), because all group Owners should be aware of the overage behaviour.

Implementation plan

create mailers	#3999 (closed)
MVC identify CustomersDot emails to send to	#4000 (closed)
send introductory email 1 day after purchase	#4007 (closed)
send introductory email 1 day after renewal	#4008 (closed)
query GitLab.com for group owners emails and send	👈 you are here

Tasks

Replace MVC solution of customers to send to with group Owners of namespace