Add push notifications for new closures

[mileswwatkins]

High-priority closures could be transmitted to all hikers on a trail, all hikers in a specific state (or who've recently messaged from a neighboring state), or all hikers who are going in that direction (would need to maintain a user database indicating NOBO or SOBO and their most recent check-in)

[mileswwatkins]

Regardless, this'll require a cache/diff/database of which closures have been seen already, so that new ones can be discerned