Migrate to JavaScript ES6

[sruest]

Move our JavaScript platform to ES6. Will involve these tasks (and possible others):

• [ ] Make all JavaScript files compatible with ES6 (import, export, etc)
• [ ] Move build to NPM, including add Maven plugin so build changes are seamless (we will still run a full build via mvn clean install)
• [ ] Investigate and configure how best to setup environment for developers. This might be using a Maven "watcher" for automatically triggering WebPack/NPM builds for changed JS files. If this is happening continuously, developers won't have to change their workflow. They will just run nunaliit update as before.
• [ ] Update/add eslintrc.json file to root project directory that is ES6 compliant.
• [ ] Thorough testing! Try upgrading a full featured atlas to ensure these changes don't break anything.
• [ ] Write an explanation in Release Notes about how to migrate ES5 to ES6. This will be helpful for anyone merging in these changes to their working branches.

[ahayes]

I would avoid a watcher in favour of having nunaliit update optionally (when detect that it is running in source tree or via a command argument) run the necessary mvn instruction to re-pack the js before the normal upload flow.