Fix epic subscriptions not working for subgroups

The change in https://gitlab.com/gitlab-org/gitlab/merge_requests/22872
caused the wrong path to be passed to the GraphQL endpoint. The frontend
passed only the immediate path (e.g. `charts`) instead of the full path
(e.g. `gitlab-org/charts`).

Closes https://gitlab.com/gitlab-org/gitlab/issues/197483

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

@@ -189,7 +189,7 @@ export const toggleEpicSubscription = ({ state, dispatch }) => {
       variables: {
         epicSetSubscriptionInput: {
           iid: `${state.epicIid}`,
-          groupPath: state.groupPath,
+          groupPath: state.fullPath,
           subscribedState: !state.subscribed,
         },
       },

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

@@ -3,6 +3,7 @@ export default () => ({
   endpoint: '',
   updateEndpoint: '',
 
+  fullPath: '',
   groupPath: '',
   markdownPreviewPath: '',
   labelsPath: '',

ee/spec/javascripts/epic/store/actions_spec.js

@@ -777,6 +777,9 @@ describe('Epic Store Actions', () => {
       },
     };
     const stateSubscribed = {
+      epicIid: 123,
+      groupPath: 'charts',
+      fullPath: 'gitlab-org/charts',
       subscribed: false,
     };
 
@@ -813,6 +816,19 @@ describe('Epic Store Actions', () => {
           ],
           done,
         );
+
+        const tester = {
+          asymmetricMatch(actual) {
+            const variables = actual.variables.epicSetSubscriptionInput;
+
+            return (
+              variables.iid === `${stateSubscribed.epicIid}` &&
+              variables.groupPath === stateSubscribed.fullPath
+            );
+          },
+        };
+
+        expect(epicUtils.gqClient.mutate).toHaveBeenCalledWith(tester);
       });
     });