This is mostly a prereq of #6655.

Goals:

• align build tooling (right now mixing grunt & gulp - commit, probably to gulp Switch entirely to gulp #4927)
• add watching, should be fast to rebuild and test changes in browser
  • could be something like webpack hot reloading
• move shared tooling back into fbjs (babel plugin, custom lint rule)

Non-goals but things to keep in mind:

• alternate build approaches, in particular flat bundling - it needs to be easy to swap this in
• releasing from master - we may want to have development happen such that feature flags are used to turn on/off features. we may want the build result to be a static flag set (no runtime checks) but would need to be possible to target any flag combination easily