[ticket/12668] Add subforums_row to core.display_forums_modify_template_vars

Add subforums template data array to
'core.display_forums_modify_template_vars' event
to allow extensions to modify it if needed.

PHPBB3-12668
Merge pull request #2553 from rxu/ticket/12668

[ticket/12668] Add subforums_row to core.display_forums_modify_template_vars event

* rxu/ticket/12668:
  [ticket/12668] Add subforums_row to core.display_forums_modify_template_vars