Fix issue #2112.

This silences the deprecation warnings for now. We should check for each
foreignkey if CASCADE is actually what we want in the future

Remove tracking pixel functionality.

Since the RegexURLPattern has no `_callback` attribute anymore the
`get_url_decorator()` wasn't called anymore.

Future tasks is to dig into self.assertRedirectsTo wouldn't work with the test
as it stands

This doesn't work since this overrides existing model fields.

Method get_fields_with_model() isn't available in Django 1.10 so replace with
_get_fields().

Use fix used on Wagtail https://github.com/torchbox/wagtail/pull/2736 to avoid
using SubfieldBase for custom fields

Django 1.10 errors (correctly) on this

Explicit pip caching not needed on Travis any more

There was a section in our .travis.yml to force Travis to cache, but this is not necessary any more as https://github.com/travis-ci/travis-ci/issues/3239 has been closed.

Update Tangent contacts for commercial support.

Fix Customizing the Base Dashboard App

Allow BankcardExpiryMonthField to have required=False

Update key_questions.rst