[back] feat: harmonization of all automatic emails sent by the API

[GresilleSiffle]

Context

To address the issue #1677 we created an email with a light CSS style that represents the Tournesol identity (see #1884).

We could easily and quickly adapt all other emails sent by the API, like the registration confirmation, password reset, and email change, to match this new style.

Proposition

All automatic emails should:

• display a white primary title on a blue background
• use the fonts helvetica, sans-serif
• be signed by the "The Tournesol team"?

Example: