Leverage the font-display CSS feature (#759)

Closes #689

Author: oliverpool

stylesheet/imports.css

@@ -16,4 +16,4 @@
  * CSS imports for FirebaseUI Javascript widget.
  */
 
-@import url(https://fonts.googleapis.com/css?family=Roboto:400,500,700);
+@import url(https://fonts.googleapis.com/css?family=Roboto:400,500,700&display=swap);