Convert Learning to frontend-base

[arbrandes]

Description

Convert the Learning 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

• Convert internal app structure (site.config, slots, etc)
• Port header and footer customizations, if any (custom menus?)
• Fix tests
• Publish to NPM
• Include in frontend-template-site
• Include in tutor-mfe as a frontend app