Collection Permissions (#829)

see https://github.com/outline/outline/issues/668

server/policies/collection.js

@@ -1,5 +1,6 @@
 // @flow
 import policy from './policy';
+import { map } from 'lodash';
 import { Collection, User } from '../models';
 import { AdminRequiredError } from '../errors';
 
@@ -11,12 +12,27 @@ allow(
   User,
   ['read', 'publish', 'update', 'export'],
   Collection,
-  (user, collection) => collection && user.teamId === collection.teamId
+  (user, collection) => {
+    if (!collection || user.teamId !== collection.teamId) return false;
+
+    if (
+      collection.private &&
+      !map(collection.users, u => u.id).includes(user.id)
+    )
+      return false;
+
+    return true;
+  }
 );
 
 allow(User, 'delete', Collection, (user, collection) => {
   if (!collection || user.teamId !== collection.teamId) return false;
-  if (user.id === collection.creatorId) return true;
+
+  if (collection.private && !map(collection.users, u => u.id).includes(user.id))
+    return false;
+
   if (user.isAdmin) return true;
+  if (user.id === collection.creatorId) return true;
+
   throw new AdminRequiredError();
 });