Pull #579 reifies the cred explorer state as an explicit state
transition machine, with a well-tested implementation. This pull
re-writes `credExplorer/App.js` to use that implementation, and thoroughly
tests it.
The result is that `credExplorer/App.js` has much simpler code
(because it just binds the rendered components to the state machine),
and is much more thoroughly tested.
To ensure easy testability of the `App` class, it was refactored so that
the module exports a factory function which takes a method for creating
the `AppStateTransitionMachine` and returns an `App` class. This ensures
that in test code, we can easily mock the state transition machine. This
had no effect on external callers, since the higher-level `<AppPage>`
class, which already wraps over `LocalStore` choice, was already the
preferred call site.
I also added a loading indicator component, which presently displays a
status text corresponding to the state, such as "Loading graph...", or
"Error while running PageRank". This way, there is always some user
feedback during loading states (which could take a while).
Test plan:
Visual inspection, and the very thorough included unit tests.
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