Lexical: Added about button/view

Re-used existing route and moved tinymce help to its own different
route. Added test to cover.
Added new external-content block to support in editor UI.

resources/js/wysiwyg/ui/framework/blocks/external-content.ts

@@ -0,0 +1,29 @@
+import {EditorUiElement} from "../core";
+import {el} from "../../../utils/dom";
+
+export class ExternalContent extends EditorUiElement {
+
+    /**
+     * The URL for HTML to be loaded from.
+     */
+    protected url: string = '';
+
+    constructor(url: string) {
+        super();
+        this.url = url;
+    }
+
+    buildDOM(): HTMLElement {
+        const wrapper = el('div', {
+            class: 'editor-external-content',
+        });
+
+        window.$http.get(this.url).then(resp => {
+            if (typeof resp.data === 'string') {
+                wrapper.innerHTML = resp.data;
+            }
+        });
+
+        return wrapper;
+    }
+}