Update Jira importer apollo client cache to be immutable

This is done in preparation for upgrading to Apollo Client 3 which
requires immutable cache updates.

app/assets/javascripts/jira_import/index.js

@@ -6,7 +6,7 @@ import App from './components/jira_import_app.vue';
 
 Vue.use(VueApollo);
 
-const defaultClient = createDefaultClient();
+const defaultClient = createDefaultClient({}, { assumeImmutableResults: true });
 
 const apolloProvider = new VueApollo({
   defaultClient,

app/assets/javascripts/jira_import/queries/initiate_jira_import.mutation.graphql

@@ -2,7 +2,6 @@
 
 mutation($input: JiraImportStartInput!) {
   jiraImportStart(input: $input) {
-    clientMutationId
     jiraImport {
       ...JiraImport
     }

app/assets/javascripts/jira_import/utils/cache_update.js

+import produce from 'immer';
 import getJiraImportDetailsQuery from '../queries/get_jira_import_details.query.graphql';
 import { IMPORT_STATE } from './jira_import_utils';
 
@@ -13,22 +14,16 @@ export const addInProgressImportToStore = (store, jiraImportStart, fullPath) =>
     },
   };
 
-  const cacheData = store.readQuery({
+  const sourceData = store.readQuery({
     ...queryDetails,
   });
 
   store.writeQuery({
     ...queryDetails,
-    data: {
-      project: {
-        ...cacheData.project,
-        jiraImportStatus: IMPORT_STATE.SCHEDULED,
-        jiraImports: {
-          ...cacheData.project.jiraImports,
-          nodes: cacheData.project.jiraImports.nodes.concat(jiraImportStart.jiraImport),
-        },
-      },
-    },
+    data: produce(sourceData, draftData => {
+      draftData.project.jiraImportStatus = IMPORT_STATE.SCHEDULED; // eslint-disable-line no-param-reassign
+      draftData.project.jiraImports.nodes.push(jiraImportStart.jiraImport);
+    }),
   });
 };