Use existing services to create epic links

- When the reordered object changes parent we need
to use the existing IssuableLinks::CreateService
corresponding to the moved object class
- Adjust permission to check that the user can
admin_epic_link

ee/app/services/epics/tree_reorder_service.rb

@@ -26,19 +26,26 @@ module Epics
     def set_new_parent
       return unless new_parent && new_parent_different?
 
-      moving_object.parent = new_parent
-      validate_new_parent
+      service = create_issuable_links(new_parent)
+      return unless service[:status] == :error
+
+      service[:message]
     end
 
     def new_parent_different?
       params[:new_parent_id] != GitlabSchema.id_from_object(moving_object.parent)
     end
 
-    def validate_new_parent
-      return unless moving_object.respond_to?(:valid_parent?)
-      return if moving_object.valid_parent?
+    def create_issuable_links(parent)
+      service, issuable = if moving_object.is_a?(Epic)
+                            [EpicLinks::CreateService, moving_object]
+                          elsif moving_object.is_a?(EpicIssue)
+                            [EpicIssues::CreateService, moving_object.issue]
+                          end
+
+      return unless service.present?
 
-      moving_object.errors[:parent]&.first
+      service.new(parent, current_user, { target_issuable: issuable }).execute
     end
 
     def move!
@@ -110,6 +117,10 @@ module Epics
       if new_parent
         return false unless can?(current_user, :admin_epic, new_parent.group)
         return false unless moving_object.parent && can?(current_user, :admin_epic, moving_object.parent.group)
+
+        if moving_object.is_a?(Epic)
+          return false unless can?(current_user, :admin_epic_link, moving_object.parent.group)
+        end
       end
 
       true

ee/spec/services/epics/tree_reorder_service_spec.rb

@@ -215,6 +215,10 @@ RSpec.describe Epics::TreeReorderService do
           let!(:tree_object_1) { epic1 }
           let!(:tree_object_2) { epic2 }
 
+          before do
+            stub_licensed_features(epics: true, subepics: true)
+          end
+
           context 'when user does not have permissions to admin the previous parent' do
             let(:other_group) { create(:group) }
             let(:other_epic) { create(:epic, group: other_group) }
@@ -260,6 +264,19 @@ RSpec.describe Epics::TreeReorderService do
             it_behaves_like 'error for the tree update', 'You don\'t have permissions to move the objects.'
           end
 
+          context 'when new parent is another epic and subepics feature is disabled' do
+            let(:other_epic) { create(:epic, group: group) }
+            let(:new_parent_id) { GitlabSchema.id_from_object(epic) }
+            let(:epic3) { create(:epic, parent: other_epic, group: group) }
+            let(:tree_object_2) { epic3 }
+
+            before do
+              stub_licensed_features(epics: true, subepics: false)
+            end
+
+            it_behaves_like 'error for the tree update', 'You don\'t have permissions to move the objects.'
+          end
+
           context 'when moving is successful' do
             it 'updates the links relative positions' do
               subject