Don't auto-suspend recently unsuspended users.

**Note:** This is an alternate implementation to #268 that uses an explicit column on the users table to record when the user was unsuspended.  This has the advantage of allowing the use of a scope to filter the users.

Additionally, don't send them spurious reminder emails.

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.