[ticket/12346] Add Template Event overall_header_navlink_append/prepend

Adding an event between `BEGIN navlinks` and `END navlinks` allows us to insert custom code in each navlink. This is needed for situations where we want to add extra content, such as drop-down menus to the navlinks.

I decided to use the singular instead of plural name, because it applies to each individual navlink, instead of the navlinks as a whole.

Example use case: http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45359

PHPBB3-12346

- [x] Depends on #2241