Fix documentation data rendering

[carltongibson]

Closes #5395.

If your coreapi.Document contains a Link called data then calls to document.data in the templates would fail.

This PR adds a test case for that and a template tag to resolve the issue. (Note, it's the same fix as #4972)

[carltongibson]

ERROR: /home/travis/build/encode/django-rest-framework/tests/test_renderers.py Imports are incorrectly sorted.
isort failed: Some modules have incorrectly ordered imports. Fix by running `isort --recursive .`
ERROR: InvocationError: '/home/travis/build/encode/django-rest-framework/runtests.py --lintonly'

Running isort tests/test_renderers.py has no effect here. I need lunch. 😀

[carltongibson]

Right, some chicken pie and a sandwich later...