Add monitoring for surge prediction freshness

We must have 12 hours of continuous surge model predictions for the
check to pass.

https://www.pivotaltracker.com/story/show/74244296 [Delivers #74244296]
Refactor: factor out self.base_time to BASE_TIME
Refactor: factor out creating locations

Also bring _make_surge_predictions out of a class
Merge pull request #88 from sealevelresearch/monitoring-for-surge-prediction

Add monitoring for surge prediction freshness