Refresh system attributes so new logo can show up right after submission

We initialize a new RT::System object with current user to update UserLogo
attribute in 9d9e524 to track who makes the change, thus we need to
explicitly refresh attributes of RT->System.

share/html/Admin/Tools/Theme.html

@@ -382,6 +382,7 @@ <h2><&|/l&>Customize the RT theme</&></h2>
             colors => $colors,
         },
     );
+    RT->System->Attributes->RedoSearch;
 
     push @results, loc("Unable to set UserLogo: [_1]", $msg) unless $id;