MDLSITE-2341 Allow content deletion for accounts older than one month

Instead of blocking the content deletion, a warning is displayed.
MDLSITE-2341 Fix unit test test_old_user() to reflect the behaviour change
Merge pull request #1 from moodlehq/MDLSITE-2341-delete-old-account

MDLSITE-2341 Allow content deletion for accounts older than one month