CANVAS-283, fixed the postioning of the header background image for a non-autheticated session

https://jira.media.berkeley.edu/jira/browse/CANVAS-283

The problem is that when it is an unauthenticated session, Canvas adds a no-user class to the header that pushes the header image up 25 px. Their non-logged in header is thinner. If we made ours thinner it would cramp the logo. 

The fix is to explicitly position the header image for both the auth and non-auth case.

#header {
  background: #003262 url(/canvas/bcourses_background_2628x72.png) no-repeat 0 0;
}
#header.no-user {
  background-position: 0 0;
}