material: replace display: flex with block in print mode

This change is a workaround for a reported issue with printing flex
items on Firefox

See: https://bugzilla.mozilla.org/show_bug.cgi?id=939897

Fixes #4251

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

app/Resources/static/themes/material/css/media_queries.scss

@@ -187,3 +187,9 @@
     display: none;
   }
 }
+
+@media only print {
+  body {
+    display: block;
+  }
+}