Refactor SubscriptionWorkPage/Subscription Vuex Module

Summary

The Subscription work page and the corresponding Vuex module are one the most important parts of the application but a frequent source of bugs, e.g. #144 (closed). The interface should be functionally the same.

Todos

• the Subscription module and the Workpage should only always be concerned with the current assignment -> No prefetching!
• the whole correction workflow should be thoroughly tested by e2e tests