[ticket/11966] Add header navigation template events to Subsilver2

Add the overall_header_navigation_append / prepend template events
used in Prosilver to subsilver2 as well. This will allow extensions to
add Nav Links to Subsilver2.
http://tracker.phpbb.com/browse/PHPBB3-11966

PHPBB3-11966
[ticket/11966] Merge branch 'develop' into ticket/11966

PHPBB3-11966
Merge pull request #1809 from VSEphpbb/ticket/11966

[ticket/11966] Add header navigation template events to Subsilver2