Add password confirmation

[4lm]

There is half of an implementation in there. It's deactivated and is missing some templating. But can be used. Seems to follow this tutorial: https://www.javatpoint.com/django-user-registration-with-email-confirmation

Todos:

• [x] decide and act on #189
• [x] solve #191
• [x] re-enable deactivated email confirmation functionality
• [x] add missing templates

[4lm]

191 is waiting to be merged and #189 can be decided independently. Closing this issue.