Add Security Warning Bar for untrusted notebooks (#970)

* Add Security Warning Bar for untrusted notebooks

* Update

* Another update

* Add a snapshot test

* UX updates

* More updates

* Add tests

* Update test snapshot

* Update string

* Update security message

src/Explorer/Notebook/NotebookComponent/NotebookComponentBootstrapper.tsx

@@ -277,6 +277,10 @@ export class NotebookComponentBootstrapper {
     return selectors.notebook.isDirty(content.model as Immutable.RecordOf<DocumentRecordProps>);
   }
 
+  public isNotebookUntrusted(): boolean {
+    return NotebookUtil.isNotebookUntrusted(this.getStore().getState(), this.contentRef);
+  }
+
   /**
    * For display purposes, only return non-killed kernels
    */