Expand license API endpoint to include add-ons

Currently the API /license endpoint only includes:

{
  "starts_at": "2015-10-24",
  "expires_at": "2016-10-24",
  "licensee": {
    "Name": "John Doe",
    "Company": "Doe, Inc.",
    "Email": "john@doe.com"
  },
  "user_limit": 100,
  "active_users": 60
}

I think we also want to add the add-ons, such as GitLab Geo. Also for #990 (closed).