atc: behaviour: add flag to enable archive pipeline

#5315 - as per
#5346 (comment),
the ability to archive pipelines is now behind an api flag
`enable-archive-pipeline`.

Signed-off-by: Aidan Oldershaw <aoldershaw@pivotal.io>
Co-authored-by: James Thomson <jthomson@pivotal.io>

atc/api/pipelineserver/archive_test.go

@@ -92,4 +92,25 @@ var _ = Describe("Archive Handler", func() {
 			Expect(body).To(Equal([]byte("endpoint is not enabled\n")))
 		})
 	})
+
+	Context("when the endpoint is not enabled", func() {
+		BeforeEach(func() {
+			server = pipelineserver.NewServer(
+				fakeLogger,
+				new(dbfakes.FakeTeamFactory),
+				new(dbfakes.FakePipelineFactory),
+				"",
+				false, /* enableArchivePipeline */
+			)
+			handler = server.ArchivePipeline(dbPipeline)
+		})
+
+		It("responds with status Forbidden", func() {
+			handler.ServeHTTP(recorder, request)
+
+			Expect(recorder.Code).To(Equal(http.StatusForbidden))
+			body, _ := ioutil.ReadAll(recorder.Body)
+			Expect(body).To(Equal([]byte("endpoint is not enabled\n")))
+		})
+	})
 })