Duplicate migration numbers can happen when merging changes from
different branches. This test ensures that we address the issue right
away.

This required extending the UnicodeCSVWriter to be able to deal with
file-like objects as well. This is used to create downloadable CSV files
in memory.

TestCase.assertEquals (note the s at the end) has been deprecated.
The only failing tests remaining are CSV related, which is not
surprising.

Tests for separator and uppercase options.

Taken from django-extensions with some adapting to our needs.

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