A new method is added to the milestone model
to support fetching milestones in this particular order:
1. Current milestones (due date > current date)
2. Milestones without due dates
3. Milestones that are expired (due date <= current_date)

The milestones are then sorted by due date (asc or desc)
and ties are broken by id in descending order.

The milestone finder is also updated to accept
a new Boolean parameter 'expired_last' to utilize the
newly added custom ordering to fetch milestones.

When 'expired_last' is specified, order param must be
one of 'due_date_asc' or 'due_date_desc'. For non-supported
order param values (including when it's not provided),
the default is 'due_date_asc'.