scopes > associations + remove dead methods

Associations are used in a few places where scopes are more
appropriate. They are both simpler and re-usable - by adding these
scopes to the Edition model, they can be used by anything that has an
:editions association, e.g. Classifications. Also, the state machine
gives us scopes for edition states for free.

I've also killed off some unused class methods on Classification.

Note that the ordering by classification_memberships.ordering on the
policies association was a red herring - this column is not being used
and editions that are associated with topics are generally ordered
explicitly using the in_reverse_chronological_order scope. The column
will be dropped in a subsequent migration.