add previous_state to event dictionary (web json)

closes #140

locklost/event.py

@@ -379,6 +379,7 @@ class LocklossEvent(object):
             'refined': self.refined,
             'refined_gps': self.refined_gps,
             'tags': self.list_tags(),
+            'previous_state': self.previous_state,
         }
 
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