[ticket/11898] Correct admin S_LOGIN_ACTION

PHPBB3-11898
[ticket/11898] Correct adm_relative_path created by installer


PHPBB3-11898
Merge remote-tracking branch 'EXreaction/ticket/11898' into develop

* EXreaction/ticket/11898:
  [ticket/11898] Correct adm_relative_path created by installer
  [ticket/11898] Correct admin S_LOGIN_ACTION