Hide new subgroup button if user has no permission to create one

Based on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10287 with few improvements.

What does this MR do?

Hides new subgroup button if user has no permission to create one

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

no

Why was this MR needed?

To avoid 404 when user tries to visit new group page without permissions

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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?

Fixes #30139 (closed)

marked the task Changelog entry added, if necessary as completed

marked the task Added for this feature/bug as completed

marked the task Conform by the merge request performance guides as completed

marked the task Conform by the style guides as completed

marked the task Branch has no merge conflicts with master (if it does - rebase it please) as completed

marked the task Squashed related commits together as completed

mentioned in merge request !10287 (merged)

added 1 commit

• 0424f3fa - Update changelog with MR id

Compare with previous version

added subgroups label

marked the task All builds are passing as completed

@rymai based on your review of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10287

assigned to @rymai

@dzaporozhets Thanks, LGTM!

merged

mentioned in commit 59f81b4f

Picked into 9-1-stable, will go into 9.1.0-rc2

mentioned in commit f4e8c965