v6.0.0

This is a major version bump since there are some breaking changes, mostly around the .env file configurations.

• Clean up webpack configurations:
  • Development: removed the need to pre-build a static development bundle
    by switching to css-hot-loader to avoid flashes of unstyled content.
    Dev environment boot up time is now much faster.
  • Production: Use the base configuration but override them depending
    on production.server (for SSR) and production.client builds.
• Refactor asset path handling as it relates to environment variables to be
  less confusing, and document it in the README.
• Clean up webpack/constants and move/rename it to config/index.js since
  the values in there are mostly application configuration level, which
  makes more sense.
• Other miscellaneous improvements.