Updates to standalone gallery (#91)

* Add hyperlink icon for links in DialogComponent

* Use a Cosmos DB image for sample Persona

* Fix tests

* Update standalone gallery behavior to match DE gallery

* Use /gallery endpoint for gallery

* Update Gallery Card style to remove buttons in standalone mode

* Address comments

* Add text banner

src/Explorer/Controls/NotebookGallery/Cards/GalleryCardComponent.test.tsx

@@ -20,6 +20,7 @@ describe("GalleryCardComponent", () => {
         views: 0
       },
       isFavorite: false,
+      showDownload: true,
       showDelete: true,
       onClick: undefined,
       onTagClick: undefined,