Several of our elements were missing z-index property, causing them to
display incorrectly in the app.
Ensure that we import styles/variables.scss before attempting to use
the z() helper function that it provides.
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.
If the total channel balance is zero, do not show the channel balance at
the top of the network view as the zero doesn't convey any useful
information and looks strange.
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