Convert Discussions to frontend-base

[arbrandes]

Description

Convert the Gradebook app to use frontend-base, as per https://github.com/openedx/frontend-base/blob/main/docs/how_tos/migrate-frontend-app.md.

The converted code will remain in a long-lived frontend-base branch.

Dropped features

The following features are removed as part of the conversion:

• ...

DEPR tickets will be created and linked above as necessary.

PRs

• ...

TODO

• Remove undesired features
• Convert basic app structure (site.config, App.tsx, etc)
• Refactor imports
• Convert header
• Convert footer
• Fix tests
• Publish to NPM as a buildless library
• Include in frontend-template-site as default configuration
• Paragon 23 (depends on frontend-base support)
• Ongoing: merges with master