New migrations need to be manually updated to correctly deal with custom
user models. This test fails if this isn't done.

This should always be used instead of Django's silent version.

This reverts fa1f8403 which moved them from models.py to __init__.py.
As reported by @mvantellingen, importing receivers from __init__.py
breaks the way models are normally overridden as it causes the core
models.py to get imported before the custom one.

We need to move back to the old system and will need to advise that the
debug toolbar needs to be set-up explicitly to avoid circular import
issues with models.

Fixes #1159
Original issue #1125

* Tidy implementation of validator
* Tests improved and moved to tests/integration (as they call resolve
  internally)

Fixes #614

So stopwords can be moved from slugs