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)

@samdutton @jiayliu PTAL
If we set onkeypress, instead of onkeydown, we can use String.fromCharCode(event.charCode) to get rid of the magic numbers.