- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

Co-authored-by: Basil Dubyk <samitnuk.work@gmail.com>
Co-authored-by: Alexander Gaevsky <sasha@sasha0.ru>
Co-authored-by: Joseph Wayodi <jwayodi@gmail.com>

The blocking dependency (Haystack) now supports Django 2.

Fixes #2526.

This reverts commit 9f74ef94e7. The related bug was fixed in Django 1.11.4

Also remove some Django 1.6 compatibility code.

Fixes #2176.

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

Replace 'assert' with 'assert*' method.

Convert tests.unit.forms.widget_tests to use TestCase class.

Convert bankcard tests to use TestCase.

Convert tests.unit.logging_tests to use TestCase.

This also removes the attr() marking of tests. This can be implemented
in pytest with marking the tests with a custom marker if required.

* Document new datetime picker in release notes.
* Update the tests to new format
* Fix some PEP8 issues

This fixes two issues:

1) The code didn't take into account that self.format will be ignored by
   Django if USE_L10N=True, unless a format was specified when constructing
   the Widget. Remedied by moving the code to render because the current
   language is determined at request time, not at object construction time,
   and looking up the correct format there.

2) The code couldn't handle date formats that include dots, like
   31.12.2014.

Fixes #1440.