Fix usage of URL_PREFIX
Currently the URL_PREFIX settings variable is used like a fully qualified URL in many parts of the website which is wrong. As the name suggests, this prefix shall essentially "move" all the URLs of the site behind it.
If a fully qualified address is needed it should be generated either using the request object associated with the method call or using Django's Sites framework.
This issue tracks the cleanup of the following:
- Email generation
- Template rendering
- Other usages