add background to #content div

it's gradient doesn't go all the way down either. We must keep
the backgrounds on #wait, #delay, and #error, since they are
needed to cover the #formWrap background.

the same background of #error is added to #content, so that
even though #error doesn't extend all the way down, the gradient
still does, and it looks correct on Android again.

fixes 1721

resources/static/css/common.css

@@ -361,7 +361,7 @@ footer .help {
   font-weight: bold;
 }
 
-#wait, #delay, #error {
+#content, #wait, #delay, #error {
   background-color: #dadee1;
   background-image: url("/i/grain.png"), -moz-linear-gradient(top, #dadee1, #c7ccd0);
 }