Move from maildev to mailpit

[Betree]

Replaces maildev by mailpit. In a nutshell:

• The server is not started automatically anymore, you need to run npm run mailpit. That would be a bit dirtier to achieve given that mailpit is not a JS/NPM package, and I figured this was ok given we don't work on emails daily.
• Mailpit has a great UI, really useful tools (like HTML compatibility checks), and a nicer integration with Cypress
• Updates managed by renovate using a custom mapper

[hdiniz]

Its also available as a docker container, in the interest of security and isolation it seems more suitable than installing a binary that run as the current user. And it works on all platforms.
https://mailpit.axllent.org/docs/install/docker/

[Betree]

Good point, I'm up for it but I don't know if everyone on the team is comfortable with Docker. Will check with the others.