Add "allowed to create subgroups" dropdown to group settings form

app/views/groups/settings/_permissions.html.haml

@@ -20,6 +20,7 @@
     = render_if_exists 'groups/settings/ip_restriction', f: f, group: @group
     = render 'groups/settings/lfs', f: f
     = render 'groups/settings/project_creation_level', f: f, group: @group
+    = render 'groups/settings/subgroup_creation_level', f: f, group: @group
     = render 'groups/settings/two_factor_auth', f: f
     = render_if_exists 'groups/member_lock_setting', f: f, group: @group

app/views/groups/settings/_subgroup_creation_level.html.haml

+.form-group
+  = f.label s_('SubgroupCreationLevel|Allowed to create subgroups'), class: 'label-bold'
+  = f.select :subgroup_creation_level, options_for_select(::Gitlab::Access.subgroup_creation_options, group.subgroup_creation_level), {}, class: 'form-control'

lib/gitlab/access.rb

@@ -110,6 +110,13 @@ module Gitlab
       def project_creation_level_name(name)
         project_creation_options.key(name)
       end
+
+      def subgroup_creation_options
+        {
+          s_('SubgroupCreationlevel|Owners') => OWNER_SUBGROUP_ACCESS,
+          s_('SubgroupCreationlevel|Maintainers') => MAINTAINER_SUBGROUP_ACCESS
+        }
+      end
     end
     def human_access