Add Custom Footer HTML (#1315)

* Add Custom Footer HTML

Straight copy from Custom Header HTML

* Move Custom Footer HTML to exactly before `</body>` tag.

* Fix invalid class name

* Append CustomFooterHTML when preparing the view.

* Some consistency in placing the variable

js/admin/src/components/EditCustomFooterModal.js

@@ -0,0 +1,24 @@
+import SettingsModal from 'flarum/components/SettingsModal';
+
+export default class EditCustomFooterModal extends SettingsModal {
+  className() {
+    return 'EditCustomFooterModal Modal--large';
+  }
+
+  title() {
+    return app.translator.trans('core.admin.edit_footer.title');
+  }
+
+  form() {
+    return [
+      <p>{app.translator.trans('core.admin.edit_footer.customize_text')}</p>,
+      <div className="Form-group">
+        <textarea className="FormControl" rows="30" bidi={this.setting('custom_footer')}/>
+      </div>
+    ];
+  }
+
+  onsaved() {
+    window.location.reload();
+  }
+}