Add more margin for single selects with allow_single_deselect

@mlharvest @kenearley @stof 

This is a relatively simple change that adds more margin to the ``span`` preceding Chosen's delete control when ``allow_single_deselect: true``. This fixes an issue when option text is longer than the width of the select (as described in #1250). I had to add a class to ``selected_item`` because there's no adjacent sibling selector in CSS that works backwards.

Fixes #1250