Spotify Black buttons + Eliminate css duplication + Hide nav links

- Added Spotify-style buttons to Spotify Black theme.

- Eliminating css duplication in dark themes by introducing useInvertedSprite bool to style objects, then importing common inverse code.

- Hide the Apps, Notification, and Share links by default, and allow them to be shown via a preference setting (https://github.com/kbhomes/google-music-mac/issues/133)