Add .disabled class styling for .btn-styles

Bootstrap itself has .btn-default.disabled styled. IE8 with angularjs has some weird behaviour if anchor has disabled attribute set, so I use disabled class as a workaround and assume the .disabled class styles the buttons the same as disabled attribute would.

less/theme.less

@@ -52,6 +52,7 @@
     border-color: darken(@btn-color, 14%);
   }
 
+  &.disabled,
   &:disabled,
   &[disabled] {
     background-color: darken(@btn-color, 12%);