Read Name of any private groups

Link:          https://hackerone.com/reports/406600
By:            @ashish_r_padelkar

Details: Summary: It is possible to see name of any private groups of other companies

Description: When company creates a Private groups, it should not be visible to other users. However, if user can just guess the ID of the group, they can see the group name!!

Steps To Reproduce:

1. Just visit below url in browser and replace the value group_id parameter

https://gitlab.com/dashboard/todos?state=&utf8=%E2%9C%93&group_id=3567586

2. Name of the group will be populated in dropdown

2.This way you can actually enumerate all the private group names from GITLAB

Impact

Get all private group names

Merge request

https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2589