Feature/build tweaks webpack sass

I'm planning to use this as a base for HRM, it's great that we have our own theme now!

Looking at some build steps I thought I would do some things a little different and wanted to know if you agree. I briefly spoke to @ryota.matsumoto and it seems that there was simply not enough time to get into too much detail.

So please let me know what you think about the changes I'm proposing:

I moved the Sass build into Webpack, because I think it makes sense to take advantage of that ecosystem and because it combines otherwise separate builds. A result of that is a combined build folder for CSS and JS. I've also added some npm scripts for tasks like fixing ESLint issues and added npm-run-all to make it easy to run multiple npm script in sequence or parallel. The Webpack, patternlab server and browsersync now are all started from a single command. There are still some tweaks I have to and would like to make in terms build output for production and the folder structure, since I find it a bit confusing how some folders are inside source, some are not, and then they get copied around for patternlab. I'd be happy about some clarification around that.

Are there other things that you've changed while working on the DC Corp site that you wanted to backport?

Feel free to make comments in the code to discuss any questions, clarifications etc.