api: Create binding to get subscription status to a stream.

Creates a binding to the endpoint
'/users/{user_id}/subscriptions/{stream_id}' that returns whether
or not a user is subscribed to a stream.

Author: agrawal-d

src/api/index.js

@@ -42,6 +42,7 @@ import subscriptionRemove from './subscriptions/subscriptionRemove';
 import toggleMuteStream from './subscriptions/toggleMuteStream';
 import togglePinStream from './subscriptions/togglePinStream';
 import toggleStreamNotifications from './subscriptions/toggleStreamNotifications';
+import getSubscriptionToStream from './subscriptions/getSubscriptionToStream';
 import unmuteTopic from './subscriptions/unmuteTopic';
 import tryGetFileTemporaryUrl from './tryGetFileTemporaryUrl';
 import createUserGroup from './user_groups/createUserGroup';
@@ -97,6 +98,7 @@ export {
   muteTopic,
   subscriptionAdd,
   subscriptionRemove,
+  getSubscriptionToStream,
   toggleMuteStream,
   togglePinStream,
   toggleStreamNotifications,

src/api/subscriptions/getSubscriptionToStream.js

@@ -0,0 +1,15 @@
+/* @flow strict-local */
+import type { Auth, ApiResponseSuccess } from '../transportTypes';
+import { apiGet } from '../apiFetch';
+
+type ApiResponseSubscriptionStatus = {|
+  ...ApiResponseSuccess,
+  is_subscribed: boolean,
+|};
+
+export default (
+  auth: Auth,
+  userId: number,
+  streamId: number,
+): Promise<ApiResponseSubscriptionStatus> =>
+  apiGet(auth, `users/${userId}/subscriptions/${streamId}`);