Subscription list rework

Description / Overview

The current way of creating/accessing subscription is confusing and overly complicated. The whole subscription abstraction should be handled behind the scenes, invisible to the user. Proposal:
The subscription list component is replaced by a card (in similar size) that has 2 (or 3 for the reviewer) tabs, corresponding to the create, validate, conflict stages. Those tabs will by default show a Random list item as well as one for each submission type. Student subscriptions are dynamically added should there be any.
On the users first login, the frontend will create Subscriptions for all available combinations of submission types and stages. When a user clicks a submission item in the list, assignments are fetched like usual. When changing the subscription the user is working on or logging out, all unfinished assignments are deleted. This will reduce the amount of 'blocking' assignments on users that are not actively working on them.

Use cases

• keep the subscription abstraction away from the user
• reduce amount of blocking assignments that are not worked on
• streamline process of getting work

Links / references

Feature checklist

• rework subscription display
• handle automatic assignment creation / deletion

(Make sure these are completed before closing the issue, with a link to the relevant commit.)