Add weather warnings

I don’t know if either of the current backends support NWS alerts or their non-US equivalents. I don’t see any indication of it from a brief look at the site, but I could have missed something.

On the front end this could just display as a little yellow or red exclamation point before the description of the weather (so it might say ! Heavy snow instead of just Heavy snow), perhaps with the color dependent on the severity of the alert.