[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
Subsilver2 should have `<!-- EVENT overall_footer_breadcrumb_prepend -->` aswell. Is it already present?