Add Select2 breadcrumbs CSS customizations in separate stylesheet
Customize Select2 image locations in CSS
Apply Select2 to all selects in Pootle

This should fix bug #2608.

Added js-select2 class to all selects in order to apply Select2.
The width styling is applied using CSS.
Added placeholders with help for multiple selects.
Removed help for multiple select in order to avoid misleading users.