Don't auto-suspend recently unsuspended users.

We are currently having issues with the workflow for unsuspending a user
who has been auto-suspended.  Currently when the user is unsuspended by
an admin, the auto-suspension process will re-suspend them at 5am the
following day if they haven't logged in by then.  This is a problem for
many users as they may work part-time, or in a different time zone, and
therefore not be able to login within that period.

This adds a 3 day grace period after unsuspension during which time the
account will not be auto-suspended.

This is currently implemented using the event log.  I'd appreciate thoughts on this approach as opposed to adding another `DateTime` field to the user model to capture the unsuspension time.
I've tweaked the index in 8c5db85