Add responsive email format to the babylon-notifier

[makirill]

MJML is a framework to generate responsive emails (more details at https://mjml.io/)

There are two major changes needed to introduce MJML framework to the babylon-notifier:

1. mjml cli needs to be installed, so it can be used by the operator.py script
2. Jinja2 templates should generate valid mjml format that will be used to generate final HTML for the email notifications.

I did the "success" path testing (provisioning start -> environment ready -> environment deleted). Deployment was done with odo tool.