Update to Django 2.*

[Feliix42]

Currently, Django 1.8.* is used. But as this version is already almost 2 years old, we should consider updating to a more recent version, either 2.0.* or 2.1.*. This can be done according to the Upgrade Instructions.

[mhaehnel]

Since beginning of August we run on Django 1.11 successfully.

While Django 1.11 has LTS until 2020 it would be nice to be prepared once that runs out. Currently I see a few blockers (such as deschler/django-modeltranslation#472) that could prevent us from upgrading.

I do not know if we need to update anything within the application to be compatible to Django 2.x

[Feliix42]

This fas been fixed in d354cdd :)