Separation of concerns: use a loading controller

Two different events can be used: `show-loading` and `hide-loading`.

I'm not 100% sure this is the right way to go, but let's try!

app/webpacker/controllers/loading_controller.js

@@ -0,0 +1,22 @@
+import ApplicationController from "./application_controller";
+
+export default class extends ApplicationController {
+  connect() {
+    super.connect();
+    document.addEventListener("show-loading", this.showLoading);
+    document.addEventListener("hide-loading", this.hideLoading);
+  }
+
+  disconnect() {
+    document.removeEventListener("show-loading", this.showLoading);
+    document.removeEventListener("hide-loading", this.hideLoading);
+  }
+
+  hideLoading = () => {
+    this.element.classList.add("hidden");
+  };
+
+  showLoading = () => {
+    this.element.classList.remove("hidden");
+  };
+}