Clean up app.current, app.previous in JS (#2156)

- Encapsulate app.current, app.previous in PageState objects
- Reorganize Page classes to use one central base class in common

Co-authored-by: Franz Liedke <franz@develophp.org>

js/src/common/components/Page.js

@@ -0,0 +1,34 @@
+import Component from '../Component';
+import PageState from '../states/PageState';
+
+/**
+ * The `Page` component
+ *
+ * @abstract
+ */
+export default class Page extends Component {
+  init() {
+    app.previous = app.current;
+    app.current = new PageState(this.constructor);
+
+    app.drawer.hide();
+    app.modal.close();
+
+    /**
+     * A class name to apply to the body while the route is active.
+     *
+     * @type {String}
+     */
+    this.bodyClass = '';
+  }
+
+  config(isInitialized, context) {
+    if (isInitialized) return;
+
+    if (this.bodyClass) {
+      $('#app').addClass(this.bodyClass);
+
+      context.onunload = () => $('#app').removeClass(this.bodyClass);
+    }
+  }
+}