Add an "Edit Mode" to Dashboard view (#3940)

* Add a togglable edit mode to dashboard

* Submenu for controls

* Allowing 'Save as' outside of editMode

* Set editMode to false as default

superset/assets/javascripts/dashboard/components/DashboardContainer.jsx

@@ -16,6 +16,7 @@ function mapStateToProps({ charts, dashboard }) {
     refresh: dashboard.refresh,
     userId: dashboard.userId,
     isStarred: !!dashboard.isStarred,
+    editMode: dashboard.editMode,
   };
 }