Use HTML emails for communication

**Is your feature request related to a problem? Please describe.**
All emails sent by Alf.io are in plain text which lacks structure and looks poor from a user perspective

**Describe the solution you'd like**
As communication is an important part of the platform, I suggest to move to HTML emails (HTML with plain text alternative)

**Describe alternatives you've considered**
In order to have responsive emails, a framework like [MJML](https://mjml.io/) could be used.