Put Select2 breadcrumbs CSS customizations in a separate stylesheet

This will allow keeping select2.css closer to upstream one.
Adjust custom Pootle colors in CSS for Select2
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.
Hide help for multiple select in order to avoid misleading users.