ReFACToring project structure

## Epic Feature
### User Story
As the product owner I want that any new project can have access to the existing interpretations of a source so i can reuse previous interpretations of a sourcepart

### Description
Outline can be found at #201

Current state:
Flint expression (Act/Fact/Duty) sits in a project

Future state:
- Flint expressions must be linked to a source
- Projects contain a set of Flint expressions
- (Performance for 3000 + "facts")

ToDo:
- Solution needed for multiple interpretations of the same nodes
- Validity/Versioning for each line in a source - linked to the validity of the law
- Collaboration between interpreters
- How to dispute the differences between interpretations