Rough extension management implementation

js/admin/src/components/AddExtensionModal.js

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+import Modal from 'flarum/components/Modal';
+
+export default class AddExtensionModal extends Modal {
+  className() {
+    return 'AddExtensionModal Modal--small';
+  }
+
+  title() {
+    return 'Add Extension';
+  }
+
+  content() {
+    return (
+      <div className="Modal-body">
+        <p>One day, this dialog will allow you to add an extension to your forum with ease. We're building an ecosystem as we speak!</p>
+        <p>In the meantime, if you manage to get your hands on a new extension, simply drop it in your forum's <code>extensions</code> directory.</p>
+        <p>If you're a developer, you can <a href="">read the docs</a> and have a go at building your own.</p>
+      </div>
+    );
+  }
+}