[ticket/11433] Rename CSS class "jalert" to "phpbb_alert"

This missing change caused the loading info to appear during ajax
requests. With this patch it will be properly hidden as it should.

PHPBB3-11433
Merge remote-tracking branch 'remotes/Marc/ticket/11433' into develop

# By Marc Alexander
# Via Marc Alexander
* remotes/Marc/ticket/11433:
  [ticket/11433] Rename CSS class "jalert" to "phpbb_alert"