[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.

PHPBB3-13026
Merge pull request #2915 from rxu/ticket/13026

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