refactor models so that we have a set of simple base models that can be used for new modules

[braaar]

As seen in issues such as #425, there is a desire for a new kind of business logic. It can be hard to implement these features without breaking existing features, so for that reason I suggest we do some refactoring work that will make it easier to make the app more modular. We can do this by writing base classes which contain the most general fields, and having the current models expand on them.

Eventually we can implement alternative versions of the business logic that uses a separate set of classes that expand the very same base models. If we write views, emails and other generalizeable code to use the base model, it will work for all use cases.