Fix for over-frequent tick timer callbacks

Right now the watchface subscribes to the tick timer at ```SECOND_UNIT``` granularity and leaves it at that. Given that (beyond the initial loading animation), the watchface is only updated every minute, this seems unnecessary. I'm not sure quite how much of a battery drain this is, but better to err on the side of caution, I suppose.

Anyway, all this does is unsubscribe from the second-timer once the loading animation has completed and changes it to a minute-timer.

The only possible caveat: when ```request_weather()``` is called after 15 minutes, the display won't be updated with the new weather data until the *next* timer tick, 60s later. If this is a real concern, it'd be possible to set the timer back to 1s refreshing while the weather data is being loaded, but I figured this would introduce unnecessary extra logic (and an opportunity for me to bug something out)...