Added ability to skip song playing.

Added ensureRoom() to server-side connection.
Clears playback when left room.
Send current playback to client upon connection.

Sends song-start timestamp to keep playback in sync.
Fix border radius on first/last column sections.
DJs are rotated, but this isn't reflected on page.
Server sends elapsed ms for playback.

This keeps playback in sync even if clients' clock's seconds are off.
DJ list updates on dj order change.
Fixed default room name displayed upon load.
Server plays next song upon song finish.