Support multilingual edition summaries.

I've had to hand-roll a migration to move the summary data onto the
`edition_translations` table, since Globalize3 doesn't seem to provide a
way to do this. The values in `editions.summary` are moved into the
corresponding english locale record in `edition_translations` table.
I've chosen to set the `updated_at` value in that table to be the
greater of `updated_at` on either the parent edition record or the
english locale record, which seems like the best we can do.

I've introduced `I18nHelpers#with_locale` because
`Globalize#with_locale` does not rollback the locale if an exception
occurs. This is very undesirable in tests, because if an exception is
raised (e.g. by `ActiveRecord::Base#save!` when validation fails) then
the different locale is then used for *all* subsequent tests.