Overhaul alert URLs for current NWS usage

Overhaul alert URLs for current NWS usage

NWS switched to using FIPS based county designations for flood
warnings around 2016, so fix the URLs for them. Also flash flood
statements and warnings (but nor watches which, for some reason,
still use WX weather zones). Severe weather statements too (but not
special weather statements nor urgent weather messages). Oh, and
thunderstorms.

Add a new tornado alert type with relevant URLs, and fix
configuration examples which reference the older tornado_warning
field which hasn't been available for years.

While at it, remove the separate flood statement alert which seems
to be entirely unused by NWS.