Create action creators and reducer for Assessment Content

[emilyparkes]

We will now be implementing Redux! 🎉😅

We will need a reducer that will manage the state of Assessment Content. To check the state changes as we expect we may need to use mock data and Redux Dev Tools.

Tasks:

• [ ] Check the wiki for relevant information.
• [ ] Create the Assessment Status reducer
• [ ] Create action creators
• [ ] Write tests for the reducer.
• [ ] Write tests for the actions.