✨ (backend) Mail reminder upcoming debit installment payment

[jonathanreveille]

Purpose

This PR will solve this issue

We need to inform the user that an upcoming installment will occur on his payment schedule.

The reminder frequency (in days) will be a configuration setting. By default, it will be 2 days.

Proposal

• [x] Create MJML template for the reminder of an installment debit
• [x] Create a debug view for our fellow developers to see the layout and the rendering of the email reminder
• [ ] Create a django command to trigger email Note : use the new method made by JB by adding a parameter to it. This command will be execute with a celery task and cron.