Use 1080p background image.

This makes the background image on the top page 1/3 as big and allows it to
load in roughly half the time.

PiperOrigin-RevId: 329462030

website/_sass/front.scss

@@ -1,5 +1,5 @@
 .jumbotron {
-  background-image: url(/assets/images/background.jpg);
+  background-image: url(/assets/images/background_1080p.jpg);
   background-position: center;
   background-repeat: no-repeat;
   background-size: cover;