Migrate to Webpack 4

[jpgilchrist]

I don't see it in any milestone or existing issue yet, so I figured I'd put it on your radar. It's supposed to be a lot faster 60 - 98% faster, in fact.

[rstoenescu]

It doesn't have to be on any milestone. Upgrading packages as they are available is always on my todo.
BUT, upgrade will occur when all the plugins are compatible with Webpack 4. It's just been released, so let's wait until things settle. Thanks.