[ticket/12857] Add template events breadcrumbs_before/after

All template events in the navlinks (breadcrumbs) list are currently _inside_ `<li>` elements.

This kinda limits the possibilities. I think it's a good idea to have some events outside the `<li>`

https://tracker.phpbb.com/browse/PHPBB3-12857
PHPBB3-12857
Yes. It's inside the long line (which we cannot break because it would introduce white-space ;) ).

The event exporter would have picked it up if it was missing :P