Simplest possible implementation of the referral code system

[Cifram]

This adds the Referral model to the database. The only way to create the codes at this stage is from the admin interface, which is awkward. I'll see about fixing that, if I have time, but this will work in a pinch. There's also no way to enter the codes on signup yet. That's next on my list.

There's no automated tests for this. I tried to add some, but found I couldn't run the unit tests. If we can solve that problem, I'll happily add tests. But as the site is supposed to be ready to go by Wednesday, I figured it was more urgent that I get the code in.

[Cifram]

So, I notice the .env file was added with the Heroku conversion. It was in the way of creating the .env directory needed for VirtualEnv, and .env is also in the .gitignore, so it seems like a thing that really shouldn't be there... But if it's a necessary part of working with Heroku, I may need to find a way to work around that. Maybe convince VirtualEnv to work out of a different directory?

[a-lost-shadow]

At least on Linux you can setup VirtualEnv to use a different directory: virtualenv

I normally use venv. Then again that's probably because the first tutorial I found also used that directory.