Receive a notification about the contracts alerts

[marien-probesys]

User story

As a commercial,
I want to receive a notification when a contract is about to end
So I can manage the business relationship with my customer peacefully and alert the techs

Overview

The alerts of a ticket (see https://github.com/Probesys/bileto/issues/442) must be sent in two ways:

• a email for each individual alert when they start
• a single email to summarize the ongoing alerts

A user should opt-in for these notifications.