Fix List.html Checkbox Width

This change gets rid of a span1 in flask_admin/templates/admin/model/list.html for the checkbox column and sets the width statically. This helps when the move to bootstrap 3 happens and I think improves the look of the list.

Before the change: http://imgur.com/gwuwzkM

After the change: http://imgur.com/I6oZGeo

Maybe we can do the same for the column with the edit and delete buttons?