Use unicode strings in test suite

This is a proposed change to show different styles of how we could
introduce unicode strings into the test suite. We should probably settle
on one of them.
Replace deprecated permalink decorator

Django now recommends using reverse():
https://docs.djangoproject.com/en/dev/ref/models/instances/#the-permalink-decorator
Use unicode slugifier for test suite

The test suite should test the complex scenarios where possible. While
we keep the default slugifier function unchanged, we support unicode
slugs and hence the test suite should test against that.

The unicode slugifier is a popular choice; Mozilla made it specifically
for Django.