OpenSprinkler will water in freezing conditions

[WillCodeForCats]

I already commented on #78 but I want to open an issue for this on its own due to the potential severity of running irrigation systems in freezing conditions.

My OpenSprinkler watered this morning when it was 25F outside. My grass is covered in ice and there are icicles on the fence. It's April 5th, some may call it spring, but even so where I live it's common get still get a random day or two like this early in the season. There's even snow in today's forecast. OpenSprinkler really should be smart enough to know not to water below freezing, yet it had a 77% water level in these conditions.

You can see in this screenshot that it has a current temperature of 25F in the background on the main page which contradicts the weather service low of 37F.

I might make my own PR to add a freeze lockout after I look at the code and if I have time, but I honestly thought OpenSprinkler would be smarter by default to know not to water in freezing conditions when using weather adjustments. For the moment I've made an automation in Home Assistant to disable the system below 34F, however I would prefer it be able to do that internally if it knows the current temperature as displayed on the main page.

App Version: 2.3.3
Firmware: 2.2.0 (3)
Hardware Version: OSPi