Preserve older alerts for a week.

1Weather/Network/Notifications/NWSAlertsManager.swift

@@ -201,13 +201,17 @@ public class NWSAlertsManager {
     private func merge(alerts newAlerts: [NWSAlert]) {
         //TODO: optimize
         var resultingSet = Set<NWSAlert>()
+        func notTooOld(alert: NWSAlert) -> Bool {
+            return alert.expires.addingTimeInterval(7 * 24 * 3600) >= Date() // 7 days in the past or newer
+        }
+        
         for alert in self.alerts {
-            if !alert.expired {
+            if notTooOld(alert: alert) {
                 resultingSet.insert(alert)
             }
         }
         for alert in newAlerts {
-            if !alert.expired && !resultingSet.contains(alert) {
+            if notTooOld(alert: alert) && !resultingSet.contains(alert) {
                 resultingSet.insert(alert)
             }
         }