[ticket/12688] Correctly use web root path instead of phpbb root path

PHPBB3-12688
Merge remote-tracking branch 'nickvergessen/ticket/12688' into develop-ascraeus

* nickvergessen/ticket/12688:
  [ticket/12688] Correctly use web root path instead of phpbb root path
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12688] Correctly use web root path instead of phpbb root path