Limit level of nesting for groups

What does this MR do?

Limit level of nesting to 20

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

no

Why was this MR needed?

To prevent overuse of the feature. GitLab UI cant display infinite level of nesting anyway. The number 20 was taken based on maximum nesting level of Android repo (15) + some extra backup

Does this MR meet the acceptance criteria?

• Changelog entry added not needed since feature not released
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/2772