Add jam clock to Caller Dashboard

[dhmay]

Add the jam clock to the Caller Dashboard. The primary use case is when the feed from the scoreboard (or visual contact with the scoreboard, for trackside calling) is broken and so JamStats is the only source of clock info.

Update it from the scoreboard as often as possible (meaning more often than we update the full game data).

Make it possible to hide this clock, since it will likely be slightly out of sync with the scoreboard clock.

Indicate visually when a jam is called off or when it ends at 2 minutes.

[dhmay]

Alternatively, maybe the clock should go at the top of the UI, outside of the Caller Dashboard context.

[dhmay]

One danger with this feature is that it will be extra likely to get out of sync with the scoreboard in exactly the situation when it's needed the most: when the venue is having bandwidth problems.

Clock feature should have a visual indicator of how long it's been since the clock was updated. Likely a green-yellow-red indicator. Perhaps even obscure the clock when it's been more than ~5 seconds since an update.