Update Swagger with Cell Axes

Cells now have axes which represent their visualization's viewport. This
updates the Swagger documentation to reflect this.

Things to be aware of
=====================

The form of "axes" is that of a map<string,object>, which is represented
in Swagger by an "additionalProperties" key (search for "string to model
mapping" here: https://swagger.io/specification/).

server/swagger.json

@@ -2714,7 +2714,6 @@
               "hipchat",
               "opsgenie",
               "pagerduty",
-              "pushover",
               "victorops",
               "smtp",
               "email",
@@ -3716,6 +3715,13 @@
                   "$ref": "#/definitions/DashboardQuery"
                 }
               },
+              "axes": {
+                "description": "The viewport for a cell's visualizations",
+                "type": "object",
+                "additionalProperties": {
+                  "$ref": "#/definitions/DashboardRange"
+                }
+              },
               "type": {
                 "description": "Cell visualization type",
                 "type": "string",
@@ -3797,6 +3803,22 @@
         }
       }
     },
+    "DashboardRange": {
+      "type": "object",
+      "description": "A description of a particular axis for a visualization",
+      "properties": {
+        "bounds": {
+          "type": "array",
+          "minItems": 0,
+          "maxItems": 2,
+          "description": "The extents of an axis in the form [lower, upper]. Clients determine whether bounds are to be inclusive or exclusive of their limits",
+          "items": {
+            "type": "integer",
+            "format": "int32"
+          }
+        }
+      }
+    },
     "Routes": {
       "type": "object",
       "properties": {