Extract a StimulusJS modal_controller

+ make `help_modal_link_controller` inherit from it

app/webpacker/controllers/help_modal_link_controller.js

@@ -1,15 +1,7 @@
-import { Controller } from "stimulus";
+import ModalLinkController from "./modal_link_controller";
 
-export default class extends Controller {
-  static values = { target: String };
-
-  open() {
-    let helpModal = document.getElementById(this.targetValue);
-    let helpModalController =
-      this.application.getControllerForElementAndIdentifier(
-        helpModal,
-        "help-modal"
-      );
-    helpModalController.open();
+export default class extends ModalLinkController {
+  getIdentifier() {
+    return "help-modal";
   }
 }

app/webpacker/controllers/modal_link_controller.js

@@ -0,0 +1,18 @@
+import { Controller } from "stimulus";
+
+export default class extends Controller {
+  static values = { target: String };
+
+  open() {
+    let modal = document.getElementById(this.targetValue);
+    let modalController = this.application.getControllerForElementAndIdentifier(
+      modal,
+      this.getIdentifier()
+    );
+    modalController.open();
+  }
+
+  getIdentifier() {
+    return "modal";
+  }
+}