Feature/tf+ 35 email notifications

[jacob271]

Fixes #35

This PR implements e-mail notifications for all notifications with a call to action (those listed in #35).

Further notes

• For the dev and test environment we used letter_opener which is a gem to mock e-mails. Instead of sending real emails, another tab opens with the content of the mail
• We use SendGrid since it allows us to scale email notifications easily (the free tier allows for up to 100 emails/day)
• @rgwohlbold will add a secret to the deployment which includes the ApiKey for SendGrid. The secret is then used in the deploy workflows
• After merging this PR, we should check if everything works as intended on the dev deployment

PR checklist

• dev-branch has been merged into local branch to resolve conflicts
• tests and linter have passed AFTER local merge
• localization is supported (Guide)
• another dev reviewed and approved
• if feature-Branch: Teams PO has approved (show via e.g. screenshots/screencapture/live demo)

[jacob271]

PO @ylloop-368 has approved this PR

[IsThisTheCrustyCrab-was-taken]

please wait for Jacob to merge this!

code and everything looks cool and test etc run locally but there is one thing concerning functionality which I'm not sure about wether it's working as expected or not. If that is the case, this can be merged.