[ticket/12843] Fix Mark forums read browser console error

When you click on "Mark forums read" in the browser console error occurs
ReferenceError: assignment to undeclared variable phpbbAlertTimer
ajax.js (line13)

PHPBB3-12843
Merge branch 'ticket/sumanai/12843' into develop-ascraeus

* ticket/sumanai/12843:
  [ticket/12843] Fix Mark forums read browser console error
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12843] Fix Mark forums read browser console error