Upgrade to npm v4.0.5 or switch to using yarn

[fbarl]

Supposedly due to this issue, we had a problem of having to run npm install twice, as the first run wouldn't pick up all the sub-dependencies of the library font-awesome-webpack that we use. That only became a breaking issue for some reason when merging #2058 into master, so that PR was reverted. #2089 restored the rolled-back code and moved font-awesome-webpack to devDependencies as a quick hack to the npm install issue.

However, font-awesome-webpack shouldn't really be in devDependencies, so a more proper solution should be used. @foot suggested following (from Slack conversation):

1. Upgrading to npm v4.0.5 - https://github.com/npm/npm/releases/tag/v4.0.5
2. Start using yarn - https://yarnpkg.com/
3. Actually running npm install twice in the build process
4. "align versions w/ deps? move to peerdeps? some npm voodoo"

IMO, both options 1 and 2 seem like a good way to go and they both worked well at the time of writing, but they would both imply changing the docker image.

I tried using yarn (see #2088), but CircleCI was having problems running it, supposedly due to this issue that got resolved in yarn (pre-release) version 0.18.0. Instructions on how to setup yarn with CircleCI: https://circleci.com/docs/install-and-use-yarn/

[fbarl]

Addressed by #2368.