Previously these events fired after the session had been destroyed, which
removes many of the useful properties. The ones I chose to preserve here are
the ones used by the community module mod_audit, which seems like a good
baseline.
When registration_delete_grace_period is set, accounts will be disabled for
the specified grace period before they are fully deleted.
During the grace period, accounts can be restored with the user:restore()
shell command.
The primary purpose is to prevent accidental or malicious deletion of a user's
account, which is traditionally very easy for any XMPP client to do with a
single stanza.
