Add License check for Burndown Charts

What does this MR do?

This shows a placeholder for burndown charts when the feature is unavailable.

The placeholder can be closed and that is stored in a cookie to not be to intrusive.

Are there points in the code the reviewer needs to double check?

This also adds a Spec helper to stub features. Separately in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2220

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing

Closes #2578 (closed)