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. Date on live show changes
3. Live show is on the schedule, then removed
4. Live show starts while pane is active