Test Plan:
Run `yarn lint` and `yarn travis` and observe success. Add something
that triggers a lint warning, like `const zzz = 3;`; re-run and observe
failures.
wchargin-branch: lint
Reorganize the code so that we have a single package.json file, which is at the root.
All source code now lives under `src`, separated into `src/backend` and `src/explorer`.
Test plan:
- run `yarn start` - it works
- run `yarn test` - it finds the tests (all in src/explorer) and they pass
- run `yarn flow` - it works. (tested with an error, that works too)
- run `yarn prettify` - it finds all the js files and writes to them