[ticket/13026] Add template vars array to core.viewonline_overwrite_location

Add template vars array to core.viewonline_overwrite_location event
to allow modifying/adding template vars for every user row.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13026">PHPBB3-13026</a>.
Yeah but it should be somewhat clear that this is the template data, not the user row from phpbb_users