Build enhancement - upgrade eslint and integrate to webpack build

[sjd78]

Upgrade and integrate eslint to the webpack build.

The upgrade will give us better linting. Integrating with the webpack build will show lint errors during development and optionally prevent the prod/CI build from succeeding with lint errors in place.

Originally posted by @sjd78 in #1359 (comment)

[sjd78]

Reference:

• https://github.com/webpack-contrib/eslint-webpack-plugin
• https://github.com/gajus/eslint-plugin-flowtype

[sjd78]

#1460 upgrades eslint and makes it work with a manual run (and therefore with IDE eslint plugin handling)
#1461 applies more styles to the react jsx formatting

Additional work is needed to integrate eslint to the dev server and the prod build.