Add .btn-xl styling

services/web/public/stylesheets/components/buttons.less

@@ -122,6 +122,10 @@
 // Button Sizes
 // --------------------------------------------------
 
+.btn-xl {
+  .button-size(@padding-large-vertical; @padding-large-horizontal; @font-size-h2; @line-height-large; @btn-border-radius-large);
+}
+
 .btn-lg {
   // line-height: ensure even-numbered height of button next to large input
   .button-size(@padding-large-vertical; @padding-large-horizontal; @font-size-large; @line-height-large; @btn-border-radius-large);