[Introductory email] - MVC Identify CustomersDot emails to send to

Description

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

Currently, we have a method within the customers model which fetches all the customers based on zuora_account_id. The method is a instance method.

We need a method which when called with parameter zuora_account_id and returns all customers matching the same zuora_account_id.

Implementation plan

create mailers	#3999 (closed)
MVC identify CustomersDot emails to send to	👈 you are here
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	#4009 (closed)

Proposal

Create a scope within Customer model which takes zuora_account_id and returns list of matching customers.