add missing event_type_reason to schema template

follow-up to #408

Author: thekaveman

agency/get_vehicle.json

@@ -182,7 +182,8 @@
             },
             "event_type_reason": {
               "enum": [
-                "missing"
+                "missing",
+                "decommissioned"
               ]
             }
           }

agency/post_vehicle_event.json

@@ -248,4 +248,4 @@
     }
   },
   "additionalProperties": false
-}
+}

schema/templates/common.json

@@ -174,7 +174,7 @@
               "enum": ["deregister"]
             },
             "event_type_reason": {
-              "enum": ["missing"]
+              "enum": ["missing", "decommissioned"]
             }
           }
         }