Prevent create|destroy design on archived projects

Don't allow modification of designs when a project is archived

ee/app/policies/ee/project_policy.rb

@@ -11,6 +11,7 @@ module EE
       vulnerability_feedback
       license_management
       feature_flag
+      design
     ].freeze
 
     prepended do

ee/spec/policies/design_management/design_policy_spec.rb

@@ -13,7 +13,7 @@ describe DesignManagement::DesignPolicy do
   let(:issue) { create(:issue, project: project) }
   let(:design) { create(:design, issue: issue) }
 
-  subject { described_class.new(current_user, design) }
+  subject(:design_policy) { described_class.new(current_user, design) }
 
   shared_examples_for "design abilities not available" do
     context "for owners" do
@@ -149,5 +149,18 @@ describe DesignManagement::DesignPolicy do
         it { is_expected.to be_disallowed(*design_abilities) }
       end
     end
+
+    context "when the project is archived" do
+      let(:current_user) { owner }
+
+      before do
+        project.update!(archived: true)
+      end
+
+      it "only allows reading designs" do
+        expect(design_policy).to be_allowed(:read_design)
+        expect(design_policy).to be_disallowed(:create_design, :destroy_design)
+      end
+    end
   end
 end