refactor Sourcegraph.com billing/license React components into function components

Created by: sqs

Also adds basic tests for almost all of these components.

See individual commits.

There is no intended behavior change; this is purely a refactor.

The diffs will not be particularly interesting or useful to review (because they are rote refactors). Because this code only affects the user-facing self-service payment path and the site admin license path, not anything that could affect live Sourcegraph instances or other parts of Sourcegraph.com, it is OK IMO to review these quickly.