Use class methods instead of scopes

These scopes were preventing me from running `rake db:schema:load` on an
empty database. I was receiving this error:

    Mysql2::Error: Table 'whitehall_development.editions' doesn't exist:
SHOW FIELDS FROM `editions`

I've tracked the problem down to these named scopes. I'm not 100% sure
of the cause but changing the scopes to class methods seems to fix it
right up. I suspect that the calls to `PublicationType.statistical` were
the culprit.