Force select2 fields to be full-width.

Select2 was setting the Readers field to a width of 100px, presumably because
the field was not visible at the time that the Select2 instance was
initialized. There's probably a way to fix this properly through the Select2
API, but I couldn't figure it out.

Fixes #69.

assets/js/single.js

@@ -15,6 +15,9 @@
 				placeholder: SocialPaperI18n.reader_placeholder,
 				data: CACSP_Potential_Readers
 			} );
+
+			// Select2 can't set Readers width correctly because it's hidden.
+			$( 'input.select2-search__field' ).css( 'width', '249px' );
 		}
 
 		if ( 'undefined' !== typeof bp && 'undefined' !== typeof bp.mentions.users ) {