Add notification policies and notification requests in web UI (#29433)

app/javascript/mastodon/reducers/notification_policy.js

@@ -0,0 +1,12 @@
+import { fromJS } from 'immutable';
+
+import { NOTIFICATION_POLICY_FETCH_SUCCESS } from 'mastodon/actions/notifications';
+
+export const notificationPolicyReducer = (state = null, action) => {
+  switch(action.type) {
+  case NOTIFICATION_POLICY_FETCH_SUCCESS:
+    return fromJS(action.policy);
+  default:
+    return state;
+  }
+};