Update Django.gitlab-ci.yml

What does this MR do?

This updates the Django CI config so it's easier for others in the future to get tests passing.

It also removes the redundant migrate, as Django will create a test database automatically and run all migrations on it. This functionality works this way in case you need to ever run all your migrations from scratch again (i.e server dies).

Are there points in the code the reviewer needs to double check?

I don't know how to do it, but creating a file in the pre-test steps with the DATABASES config would make this template more plug-n-play.

Why was this MR needed?

Migrate wasn't needed and it isn't clear otherwise how to get tests working