Make File/URL selector btn-secondary

This ensures you receive visual feedback when tabbing between form fields with the keyboard. At present there is no visual indication when you get to the File/URL dropdown.

binderhub/static/js/components/LinkGenerator.jsx

@@ -92,7 +92,7 @@ function UrlSelector({ setUrlPath }) {
           onChange={(e) => setPath(e.target.value)}
         />
         <button
-          className="btn border border-2 border-start-0 dropdown-toggle"
+          className="btn btn-secondary border border-2 border-start-0 dropdown-toggle"
           type="button"
           data-bs-toggle="dropdown"
           aria-expanded="false"