Fixes #1886, <button> in group does not take full width

src/definitions/elements/button.less

@@ -2384,6 +2384,7 @@
 .ui.vertical.buttons .button {
   display: block;
   float: none;
+  width: 100%;
   margin: @verticalGroupOffset;
   box-shadow: @verticalBoxShadow;
 }