Use epic iid attribute when updating epic in UI

iid attribute is expected when updating an epic in graphql API. So
we should pass iid instead of id.

ee/app/assets/javascripts/epic/store/actions.js

@@ -129,7 +129,7 @@ export const requestEpicDateSaveFailure = ({ commit }, data) => {
 };
 export const saveDate = ({ state, dispatch }, { dateType, dateTypeIsFixed, newDate }) => {
   const updateEpicInput = {
-    iid: `${state.epicId}`,
+    iid: `${state.epicIid}`,
     groupPath: state.groupPath,
     [dateType === dateTypes.start ? 'startDateIsFixed' : 'dueDateIsFixed']: dateTypeIsFixed,
   };
@@ -188,7 +188,7 @@ export const toggleEpicSubscription = ({ state, dispatch }) => {
       mutation: epicSetSubscription,
       variables: {
         epicSetSubscriptionInput: {
-          iid: `${state.epicId}`,
+          iid: `${state.epicIid}`,
           groupPath: state.groupPath,
           subscribedState: !state.subscribed,
         },

ee/app/presenters/epic_presenter.rb

@@ -61,6 +61,7 @@ class EpicPresenter < Gitlab::View::Presenter::Delegated
   def base_attributes(author_icon)
     {
       epic_id: epic.id,
+      epic_iid: epic.iid,
       created: epic.created_at,
       author: epic_author(author_icon),
       ancestors: epic_ancestors(epic.ancestors.inc_group),

ee/spec/fixtures/api/schemas/epic_meta_data.json

@@ -11,6 +11,9 @@
     "epic_id": {
       "type": "integer"
     },
+    "epic_iid": {
+      "type": "integer"
+    },
     "created": {
       "type": "string"
     },