Make subgroup_creation_level default to maintainer at SQL level

- Migration updates existing groups to "owner", then sets default to
  "maintainer" so that new groups will default to that
- Update spec examples

spec/controllers/admin/groups_controller_spec.rb

@@ -70,14 +70,13 @@ describe Admin::GroupsController do
     end
     it 'updates the subgroup_creation_level successfully' do
-      MAINTAINER = ::Gitlab::Access::MAINTAINER_SUBGROUP_ACCESS
+      OWNER = ::Gitlab::Access::OWNER_SUBGROUP_ACCESS
       expect do
         post :update,
              params: { id: group.to_param,
-                       group: { subgroup_creation_level: MAINTAINER } }
-      end.to change { group.reload.subgroup_creation_level }
-               .to(MAINTAINER)
+                       group: { subgroup_creation_level: OWNER } }
+      end.to change { group.reload.subgroup_creation_level }.to(OWNER)
     end
   end
 end

spec/factories/groups.rb

@@ -5,7 +5,6 @@ FactoryBot.define do
     type 'Group'
     owner nil
     project_creation_level ::Gitlab::Access::MAINTAINER_PROJECT_ACCESS
-    subgroup_creation_level ::Gitlab::Access::OWNER_SUBGROUP_ACCESS
     after(:create) do |group|
       if group.owner

spec/features/groups/show_spec.rb

@@ -78,10 +78,11 @@ describe 'Group show page' do
       context 'when subgroups are supported', :nested_groups do
         before do
           allow(Group).to receive(:supports_nested_objects?) { true }
-          visit path
         end
         it 'allows creating subgroups' do
+          visit path
+
           expect(page)
             .to have_css("li[data-text='New subgroup']", visible: false)
         end
@@ -90,10 +91,11 @@ describe 'Group show page' do
       context 'when subgroups are not supported' do
         before do
           allow(Group).to receive(:supports_nested_objects?) { false }
-          visit path
         end
         it 'does not allow creating subgroups' do
+          visit path
+
           expect(page)
             .not_to have_selector("li[data-text='New subgroup']", visible: false)
         end

spec/requests/api/groups_spec.rb

@@ -803,10 +803,10 @@ describe API::Groups do
           group2.add_maintainer(user1)
         end
-        it 'cannot create subgroups' do
+        it 'can create subgroups' do
           post api("/groups", user1), params: { parent_id: group2.id, name: 'foo', path: 'foo' }
-          expect(response).to have_gitlab_http_status(403)
+          expect(response).to have_gitlab_http_status(201)
         end
       end
     end