fix(editor): fetch exception dates in main GraphQL query

This allows us to do better UI-side validation of whether there are duplicate dates in the
exceptions

re #561

Author: landonreed

lib/editor/actions/editor.js

@@ -411,6 +411,9 @@ export function fetchBaseGtfs ({
           schedule_exceptions (limit: -1) {
             id
             name
+            # Fetch dates to ensure we can do validation in the UI
+            # (avoid duplicate dates).
+            dates
           }
           stops (limit: -1) {
             id