Merge branch '18-create-email-templates-and-implement-sending-from-backend' into 'main'

Implement Mail Sending & Template

- added sending mails via nodemailer  -> .env may need to be updated
- added template loading from server/src/assets
	* special email html for readability across multiple mail clients
	* expressions inside {{}} are replaced by the values of the replace argument in the sendMail function
	* templates are both in txt and html for mail clients that don't support html

Closes #18

See merge request swp-ws21-fahrtenbuch/team-einhorn/fahrtenbuch!10