[frontend-app-library-authoring] Spring cleaning

[arbrandes]

frontend-app-library-authoring hasn't seen some love in a while. It needs:

• Use a shared header component, instead of a custom one. #70
• CSS refactor #72
• Remove disallowed generic PropTypes workarounds #81
• Rewrite class-based components as functions
• Raise test coverage to a decent level, and in the process standardize on testing strategy.

[arbrandes]

@Carlos-Muniz, it's probably best if we create the sub-tasks in the frontend-app-library-authoring repo, instead of here, now that the repo has issues enabled.

[arbrandes]

Back on the backlog for anybody to take on.

[navinkarkera]

@arbrandes Is this issue still relevant?

[arbrandes]

Pretty much. The codebase could still benefit from some clean-ups.

[kdmccormick]

Closing per #439