Rework hotkeys; fill browser with video

- Use hotkeys without control chars (similar to Gmail)
- Add hotkey for fullscreen, change mute to Space
- When going fullscreen, show local video as well
- Make local video slightly smaller and add a small border
- Use object-fit: cover to stretch video to fill the browser (no
letterboxing)
Fix csslint errors
Clean up some crufty code in the action handlers
Merge pull request #255 from GoogleChrome/apprtc-polish

Rework hotkeys; fill browser with video