Added loading message to Grade Distribution

[KevinWu098]

Description

1. Instead of returning null while loading, we now return a loading message return <>Loading Distribution..</>;

Screenshots

Before:

After (search):

After (course):

Steps to verify/test this change:

• Verify changes work as expected on staging instance

Final Checks:

• Verify successful deployment
• Delete branch

(optional)

• Write tests
• Write documentation

Closes #336

[js0mmer]

LGTM

[github-actions]

Deployed staging instance to https://staging-337.peterportal.org