Upgrade to latest Django LTS version

[chrismytton]

We're currently on Django 1.8, which reached end of life April 1 2018. We should upgrade to at least 1.11 LTS, which is supported until April 2020.

[chrismytton]

Rather than trying to go all the way to 1.11 immediately, I think it will be easier to go via 1.9 and 1.10.

I've tried upgrading to Django 1.9.13 in requirements.txt and the first problem is these errors from pip install -r requirements.txt

django-info-pages 0.0.5 has requirement Django<1.9,>=1.8.12, but you'll have django 1.9.13 which is incompatible.
za-hansard 0.2 has requirement django==1.8.18, but you'll have django 1.9.13 which is incompatible.
django-sayit 1.4.1 has requirement Django<1.9,>=1.8.5, but you'll have django 1.9.13 which is incompatible.

• The tests for django-info-pages are failing, so that needs to be fixed first - Tests are failing django-info-pages#6
• Hopefully za-hansard just needs the Django version requirement loosening, but there's a lot of code in there and the tests aren't passing currently, so we might want to fix those first to give us a better safety net.
• Updating to django-sayit 1.5 should fix that problem