This commit updates eslint from v4 to v6. In doing so, I've moved off of
the create-react-app base eslint config. We were on an old version (v2)
and it doesn't make sense to update to v4, as in v4 create-react-app
uses typescript. Also, it didn't make sense to stay on
create-react-app's v2 config, because then it had unmet peer dependency
constraints on old versions of eslint.
Instead, I've moved us to use the default rules for eslint,
eslint-plugin-react, and eslint-plugin-flowtype.
I also made some changes to the codebase to satisfy the new lint rules
that came with this change.
Test plan: `yarn test` passes.
7a493b596d