William Chargin c1b37fa729 Add an end-to-end component test for App
Summary:
This should ensure that there aren’t any runtime PropTypes errors.

The change to reimplement `Object.entries` is due to the fact that my
local Node environment (v6.11.1) does not yet support that ESnext
function.

Test Plan:
Apply the following diff (`git apply`):
```diff
diff --git a/explorer/src/UserExplorer.js b/explorer/src/UserExplorer.js
index 8da252d..cdc1370 100644
--- a/explorer/src/UserExplorer.js
+++ b/explorer/src/UserExplorer.js
@@ -10,7 +10,7 @@ export class UserExplorer extends Component {
   static propTypes = {
     selectedPath: PropTypes.string.isRequired,
     selectedUser: PropTypes.string,
-    onSelectUser: PropTypes.func.isRequired,
+    onSelectUser: PropTypes.number.isRequired,
     data: commitUtilsPropTypes.commitData.isRequired,
   }
```
Then, `yarn test` should fail. Revert the diff, and `yarn test` should
pass.

wchargin-branch: react-proptypes-test
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..
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