package.json and yarn.lock are out of sync

[Hamms]

This is mostly a problem as concerns webpack, since according to the package.json we are using webpack 4 which requires node >= 6.11.

Recommended fix:

• pick a version of node and add an nvmrc (recommend something in v10, the latest LTS)
• pick a version of webpack based on the selected version of node and update both package.json and lockfile to use that version
• possibly switch to using npm >= 5 and package-lock.json over yarn