[ticket/11991] Fix incorrect class name for report_post_closed notification.

PHPBB3-11991
[ticket/11987] Change {ROOT_PATH} to point to board root.

PHPBB3-11987
[ticket/11987] Replace {PHPBB_ROOT_PATH} with {ROOT_PATH}.

PHPBB3-11987
[ticket/11987] Add {ADMIN_ROOT_PATH}.

PHPBB3-11987
Merge remote-tracking branch 'prototech/ticket/11991' into develop

* prototech/ticket/11991:
  [ticket/11991] Fix incorrect class name for report_post_closed notification.
Merge remote-tracking branch 'prototech/ticket/11987' into develop

* prototech/ticket/11987:
  [ticket/11987] Add {ADMIN_ROOT_PATH}.
  [ticket/11987] Replace {PHPBB_ROOT_PATH} with {ROOT_PATH}.
  [ticket/11987] Change {ROOT_PATH} to point to board root.