[SM, SaaS - Introductory email] - create mailers

## Description

As part of https://gitlab.com/groups/gitlab-org/-/epics/7229, we will be sending few mails 1 day after a subscription has been purchased or renewed.

We need to have a new mailer which can pick customers from a given order id and sends email to the list of customers matching order's `zuora_account_id`

We currently have few mailers already within CustomersDot. Having a new mailer would help us separate introduction emails from rest of the mailers already present. 

## Implementation plan

|  |  |
| ------ | ------ |
| create mailers | :point_left: you are here |
| MVC identify CustomersDot emails to send to | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4000 |
| send introductory email 1 day after purchase |https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4007 |
| send introductory email 1 day after renewal | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4008 | 
| query GitLab.com for group owners emails and send | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4009 |

## Proposal 

### Design for new purchase

[Design here](https://gitlab.com/gitlab-org/gitlab/-/issues/347267/designs/Email_to_group_Owners_1_day_after_purchase_with_info_about_QSR.png)

### Email copy for new purchase

#### Email copy

> Subject line: Managing users in your subscription
>
> Dear \[customer\],
>
> Thank you for purchasing a GitLab.com subscription! As you get started, we want to make sure you understand some important billing information.
>
> Your subscription cost is based on the maximum number of users enabled during the billing period. If you reach the maximum number of users in your subscription, you can continue to add users and GitLab bills you for the overage. These bills occur every quarter.
>
> Read more about quarterly reconciliation in [our product documentation](https://docs.gitlab.com/ee/subscriptions/quarterly_reconciliation.html).
>
> If you have any questions about this process, [please reach out](https://support.gitlab.com/hc/en-us/requests/new?ticket_form_id=360000071293). We are happy to assist.
>
> Thank you again for your purchase!
>
> GitLab Billing Team

### Design for renewal

[Design here](https://gitlab.com/gitlab-org/gitlab/-/issues/347267/designs/Email_to_group_Owners_1_day_after_purchase_with_info_about_QSR.png)

### Email copy for renewal

> Subject line: Managing users in your subscription
>
> Dear \[customer\],
>
> Thank you for renewing your GitLab.com subscription! As you get started, we want to make sure you understand some important billing information.
>
> Your subscription cost is based on the maximum number of users enabled during the billing period. If you reach the maximum number of users in your subscription, you can continue to add users and GitLab bills you for the overage. These bills occur every quarter.
>
> Read more about quarterly reconciliation in [our product documentation](https://docs.gitlab.com/ee/subscriptions/quarterly_reconciliation.html).
>
> If you have any questions about this process, [please reach out](https://support.gitlab.com/hc/en-us/requests/new?ticket_form_id=360000071293). We are happy to assist.
>
> Thank you for your business!
>
> GitLab Billing Team

### Implementation

1. Using `rails generate` command to create introduction mailer 
1. Add `default_to` to the new mailer.

Story points: 2.0