Factor out common definition of Tag type (#1793)

... and remove spurious `additionalProperties: true`

data/event-schemas/schema/m.tag.yaml

@@ -17,16 +17,7 @@
           "type": "object",
           "description": "The tags on the room and their contents.",
           "additionalProperties": {
-            "title": "Tag",
-            "type": "object",
-            "properties": {
-              "order": {
-                "type": "number",
-                "format": "float",
-                "description":
-                  "A number in a range `[0,1]` describing a relative position of the room under the given tag."
-              }
-            }
+            "$ref": "../../api/client-server/definitions/tag.yaml"
           }
         }
       }