Upgrade webpack dependencies

Updates webpack and all associated loaders/plugins to their latest versions.

Notable Changes:

webpack (changelog):

• switches from uglify-es to terser internally for code minification (more stable, less bugs)
• lots of bug fixes
• internal improvements
• improved rebuild performance for webpack-dev-server

webpack-cli (changelog):

• better schema validation
• support for "webpackfile"

babel-loader (changelog)

• update sub-dependencies relating to internal caching

css-loader (changelog): ( reverted due to breaking changes that interfere with monaco editor )

• changes in the way imports are resolved (see changelog)

cache-loader (changelog):

• performance improvements

webpack-bundle-analyzer (changelog)
vue-loader (changelog) ( reverted because sub-dependency breaks node 6 support )
url-loader (changelog)
style-loader (changelog)
raw-loader (changelog)
file-loader (changelog)
webpack-dev-server (changelog)