[ticket/12857] Add template events breadcrumbs_before/after

PHPBB3-12857
[ticket/12857] Add missing template events breadcrumb_prepend

To insert links after site-home

PHPBB3-12857
[ticket/12857] Clarify events.md on difference between PS and SS2 breadcrumbs

PHPBB3-12857
[ticket/12857] Add header/footer distinction for subsilver2 events

PHPBB3-12857
Merge remote-tracking branch 'paybas/ticket/12857' into develop-ascraeus