migrate mailing

[scheidtdav]

The backend needs to be able to send emails to users, just like the current osem api.

We most likely need to add new infrastructure to the docker setup and make sure that it works for our production environment as well.

Additionally some endpoints that have already been migrated need to be visited again, as they require emails to be send out:

• POST /users/request-password-reset
• POST /users/me/resend-email-confirmation
• change email
• create device
• delete user
• set language properly
• rework email content to separate senseBox
• Fix tests using email