Follow-up from "Hide `New subgroup` button if visibility is restricted"

The following discussion from !148758 (merged) should be addressed:

Implementation plan

• @jarka started a discussion: (+3 comments)

  I am wondering if this could be extracted outside of the policy class.

  Maybe we could create a new method inside Gitlab::VisibilityLevel

  Ideally, in the policy class, we should use a very simple check, for example:

  condition(:create_subgroup_disabled, scope: :subject) do
    Gitlab::VisibilityLevel.allowed_levels_for_user(@user, @subject).empty?
  end

  WDYT?