Add ducking, now playing display, multi-device control

[penrods]

Server minor and larger changes:

• Add awareness of Spotify Connect playing on other devices
• Transfer playback to local device before attempting playback
• Add 5 second "auto ducking" when wakeword is heard
• Add display of current track artist and title
• Add support for simple 'Play Spotify' command

[forslund]

A bunch of tiny things but over all feels much better. Merging now.