[FE] Restrict private project forks, either completely or to private forks only

frontend for https://gitlab.com/gitlab-org/gitlab-ce/issues/15082

For private projects, add a new element under Repository in Project Settings -> General -> Permissions:

• Title: 'Forks'
• Description: 'Allow users to make copies of your repository to a new project'
• Dropdown options:
  • 'Allow public forks'
  • 'Only allow private forks'

Switching the toggle off will disallow all forks. Switching the toggle on will give users the option between allowing all forks, or only private ones.

There will be a question icon that will show a popover on hover:

Allowing only private forks will force the visibility of new forks to Private. Existing forks will not be affected.