Fill Assignment Details on Assigning Gradeable Configuration Page

[andrewdbinder]

What problem are you trying to solve with Submitty
When a user selects a homework assignment from the library in the search results, they should see a preview in the Assignment Details panel, and should show whatever the backend team deems is available to view (ideally readme, unideally a description from the config.json).

Describe the way you'd like to solve this problem
The backend team should provide a function that returns either a markdown readme if there is one available, otherwise it should return a description from inside config.json, and if that doesn't exist, the config.json itself?

The frontend team needs to call the backend team's function and display whatever they deem is appropriate for the assignment.