ES6 migration

[simison]

For better developer experience, we should upgrade the codebase to ES6.

Some notes:

• Update eslint config to support ES6 env (done):
  • we might need to run two separate configs for different sections of the application for a short while. That's far from ideal so we should keep boundaries of ES5/6 as clear as possible (e.g. clientside/serverside).
  • For easier management of different eslint configs, move eslint out from Gulp: Refactor eslint out from Gulpfile #990
• Add Babel to Webpack build to transpile code back to ES5 for client-side files. (done)
  • Our Angular1 eslint rules might need attention at this point
• Do swipes of basic ES5-to-ES6 updates either with codemods (https://github.com/5to6/5to6-codemod) and/or with eslint --fix.
  • Smaller the PRs the easier it's to review/test/merge.
  • Good boundaries for each PR might be app modules as well as clientside/serverside. Serverside changes should probably go in first because that will work out of the box while clientside will need transpiling.
• Add Prettier for automated code formatting to keep ES5 and ES6 codebases more aligned and just generally for developer happiness (issue prettier: Opinionated Code Formatter #858, PR Add Prettier - an opinionated code formatter #932)

[simison]

FYI @nicksellen who might have experience from doing this elsewhere; how does the outlined plan look like?

FYI @mrkvon who I know is super excited about this as well. :-)

Anyone got further ideas, experiences? Anything I'm missing?

[nicksellen]

I think this can be closed, now #1126 is merged 🚀

(the open prettier topic has it's own issue)