Allow ground frost job to be run at any time.

[jskripsky]

When to do what:

• started in the morning: check past night temperatures, block if necessary
• started in the evening: check coming night temperatures, block if necessary (done)
• started between 8:00 and 17:00: do nothing

Additionally:

• first check if there's already a blocking scheduled
• remove existing blocking...
  • before adding a blocking with different duration
  • when the night was warmer then forecast