[ticket/11959] Trim the list of users from post notifications.

PHPBB3-11959
[ticket/11959] Use COMMA_SEPARATOR to join the user list.

PHPBB3-11959
[ticket/11959] Rename $extra_cnt to something more descriptive.

PHPBB3-11959
Merge pull request #1882 from prototech/ticket/11959

[ticket/11959] Trim the list of users from post notifications.