Set padding/margin for select2-results list items

This pull request explicitly sets the expected padding and margin for  `.select2-results li` elements to ensure it looks correct rather than assuming `.select2-results li` will not inherit any padding or margin.

For example, if using a `reset.css` file that adds padding and margin to `<li>` tags, then the select2 menu is inheriting this padding and margin and it looks broken.