Countdown

Implementation of #5. More of a start than anything, but it is functional. Styles are broken; haven't touched them yet.

Stuff left to do:

1. Make it look not-terrible (light theme especially)
2. Add the name of the upcoming show above the timer
3. (Maybe) add a link to the chat page once the timer is up
4. (Maybe) add a refresh button to that tab 

And some test scenarios:

1. Multiple live shows on the upcoming schedule
2. No live shows on the upcoming schedule
3. Live show on the upcoming schedule, but something is currently live
4. Date on live show changes
5. Live show is on the schedule, then removed
6. Live show starts while pane is active