Convert Template App to frontend-base

[arbrandes]

Description

Convert the Template Application 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:

(none so far)

DEPR tickets will be created and linked above as necessary.

PRs

• refactor: migrate to frontend-base #906

TODO

• Initial conversion
• Paragon 23 (depends on frontend-base support)
• Ongoing: merges with master