Fix clipped broadcast message on login page when using custom header

Broadcast message was clipped on the login page when using a custom
header. This commit adjusts some CSS to fix this issue.

Changelog: fixed

app/assets/stylesheets/pages/login.scss

@@ -206,6 +206,12 @@
   padding: 0;
   height: 100%;
 
+  &.with-system-header {
+    .login-page-broadcast {
+      margin-top: $system-header-height + $header-height;
+    }
+  }
+
   // Fixes footer container to bottom of viewport
   body {
     // offset height of fixed header + 1 to avoid scroll