Support multilingual edition body.

As in d2d5baac5680a5daf4f209abbda5287fc0f04415, I've hand-rolled the
migration with the same logic for setting the `updated_at` value on the
record in `edition_translations`.

I have removed
`StripInlineAttachmentsFromPublicationsAndConsultationsTest`, because it
was testing an old migration which has already run in production and
there didn't seem much value in updating the test to work with the
latest schema. Also I'm pretty unconvinced it's sensible to add to our
already long build time with a test for a migration which is almost
certainly never going to change. The test was originally added in
d63004a08b06aa125c7350dab6e90c04c8081cbb.