To do `GROUP_BY` queries in django requires one to use `.values()`
eg this groups posts by user getting a count of posts per user.

```
Posts.objects.order_by('user').values('user').annotate(post_count=Count('post'))
```

This would produce a value queryset which serializes its result
objects as dictionaries while `CursorPagination` requires a queryset
with result objects that are model instances.

This commit enables cursor pagination for value querysets.

- had to mangle the tests a bit to test it out. They might need
  some refactoring.
- tried the same for `.values_list()` but it turned out to be
  trickier than I expected since you have to use tuple indexes.

Since `str` objects are immutable, appending to existing `str` creates
in fact a new `str` instance.

Thus `ErrorDetail.detail.code` attribute is lost after `str` concatenation operation.

APIException has a get_full_details() method but the documentation refers to full_details().

Add `drf-proxy-pagination` reference to Pagination docs

when using with django-filter and rest_framework_swagger need to import coreapi before django-filter as django filter tries to load rest_framework.coreapi which is undefined at this point

... in section ModelSerializer 'fields' and 'exclude'

Deprecate the built-in `rest_framework.filters.DjangoFilterBackend` in favour of the third-party `django_filters.rest_framework.DjangoFilterBackend`.

Fixed minor sentence in Filter.py filter_queryset() Comment

* installation for django added in requirements.txt
* added line to install django first in contributing.md

* added line to install django first in contributing.md and CONTRIBUTING.md

Handle ModelSerializer case for relationships to models with custom pk.

When HTML form input is used with JSONField, treat the input as a JSON encoded string, not a JSON primative. (#4565)