update background on registration page. fixes #11037

test plan:
  * navigate to /register;
  * verify that the background is grey.

Change-Id: I9d32a7ed20053c1c4f9d89af5dc4b1493b844ce2
Reviewed-on: https://gerrit.instructure.com/14389
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>

app/stylesheets/registration.scss

@@ -1,7 +1,7 @@
 @import 'environment';
 
 body {
-  background: #fff url(/images/registration/blue-tile.png) 0 0 repeat;
+  background: #fff url(/images/registration/page-background.png) 0 0 repeat;
 }
 
 a {