This update uses a function to apply the z-index in each .scss file.
The function needs to iterate over a map of z-index values, so all of
the z-indexes end up in a single list and can be tracked more easily.
This allows us to manage all z-index values in a single location.
Rather than using relative paths for css imports - which can break when
we move files around - add our styles path to the webpack sass-loader
loader using the `includePaths` setting.
Tom Kirkpatrick
dfat
Jack Mallers
Ben Woosley