* Fixes issue #4067 - Wunderground sensor with alerts exceeds API limits
To avoid hitting the max limit of 500 calls per day, this patch keeps weather conditions being updated each 5 minutes
and weather advisories each 15 minutes.
This formula will result the following:
conditions -> 300 seconds -> 5 minutes -> 12 req/h -> 288 req/day
alerts -> 900 seconds -> 15 minutes -> 4 req/h -> 96 req/day
* Using timedelta in minutes instead seconds
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