feature: submissions dashboard

[jwsmith24]

With these changes, Palette is now able to fetch all submissions for an assignment from Canvas, organize the submissions into groups, and display each group (along with grading progress) in a dashboard-style interface.

If submissions don't have a group association, they're added to a group that's just called "No Group". I'm thinking eventually, we could implement an alternative dashboard view depending on if the assignment is a group assignment or individual. Since the focus is on group project grading, figured I should move on to the actual grading functionality first.

Key parts of the code that could probably benefit from some extra scrutiny:

• Backend stuff (see backend/src/services/transformers.ts)
  • Transformation logic
  • Group sorting algorithm
• Frontend stuff
  • UI feedback as you guys check the dashboard out on your own screens

[jwsmith24]

added the default rating updates into this PR since they're minimal!

Updated the text to match her request and also left-aligned the title because centered was looking odd to me. Let me know what you think!

[jwsmith24]

Andddd just added a couple tweaks to the dashboard layout to clean some stuff up. Done making changes until your review is in!