Proposal refactoring, unit test improvements, documentation updates

• Added nested route for calls in proposals (to be consistent with calls in cycles)
  • Added unit tests for this route
• Extracted 'deadline has passed' logic to function on Proposal model
  • Added unit tests for this function
• Added more unit tests for Proposal model and split up tests between different files to increase readability
• Modified some Proposal unit tests to not use database models, so that they run faster and only test what they need to test
  • Added proper mocking of datetime.now to be able to test all conditions
• Updated the data model documentation
• Renamed the Proposal call field in the API to call_id
• Add the option to generate state diagrams + proposal state diagram itself