Improve banner layout on mobile

The previous change to add a summit banner seems to have caused the
header not to span the page correctly on mobile browsers. Adjust the
mobile banner height to auto.

Change-Id: Ie1deca9ab1a7a1f85f6af063832eeb5519f1a7ec

www/assets/css/main.css

@@ -2327,7 +2327,7 @@ input[type="button"],
 
   /* Banner */
   #banner {
-    height: 18em; }
+    height: auto; }
 
   /* Wrapper */
   .wrapper {
@@ -2368,7 +2368,7 @@ input[type="button"],
 
   /* Banner */
   #banner {
-    height: 20em; }
+    height: auto; }
     #banner header {
       padding: 20px; }