Track/Display cron last-run time

Fairly minor feature, but with the ability to switch phpBB's cron tasks entirely over to a system's crontab, it'd be nice to be able to see in the admin panel when the phpBB's cron script(s) successfully ran.

This could be as simple as touching a file in the cache (rather than hitting the database) but it'd help to just confirm that things are running regularly (i.e- are properly configured).