[ticket/12347] Move breadcrumb seperator from template to CSS

It seems to work fine with the responsive breadcrumbs script. The bidi.css might need some work (although it works fine in general), but the bidi.css needs overall work. There are other issues that need to be solved first.

Lastly: before anyone asks, you cannot use HTML special characters in CSS `content:''`, so I used the display value, which is the preferred method for CSS (as far as I know).

PHPBB3-12347