WIP initialize mailing for developing and sending emails

[rschlaefli]

install the mailing framework (https://www.mailing.run/docs#whats-mailing) based on mjml-react (react-based way of defining email templates) and start incorporating it in code to send emails -> start the dev server with 'pn dev:mailing' in packages/graphql -> allows working on emails in DEV + acts as a fake mail server when sending mails from dev mode

it feels a bit "unclean" to install react and next (as a dev dep) in the backend code. however, the emails are rendered from react to html in the backend when they are sent, so this seems required. we should investigate later whether there is a better way

TODO: getting it to work with oauth 2 login for Outlook (would send from [email protected]) in production (will require ZI cooperation) or using cleverreach with SMTP (would send from [email protected], less preferable) -> separate PR TBD TODO: replacing everything with env variables and adding to deploy scripts -> separate PR TBD TODO: nice email template for generic transactional emails + specific one for magic link -> separate PR TBD