Namespace license checks for burndown charts

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/2335

Burndown charts (EES / Bronze)

1. Do not display the burndown chart on the milestone page /<groupname>/<projectname>/milestone/n

2. Hard to promote easily without being annoying as chart is included on a page. Could include a Text link saying "Upgrade your plan for burndown charts"

Introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1540 by @felipe_artur and @mikegreiling

mentioned in issue #2335 (closed)

assigned to @reprazent

mentioned in merge request !2219 (merged)

closed via merge request !2219 (merged)