Don't allow dashes in format or update_type

In order to reduce confusion require underscores to separate words in
these fields (similarly to keys in the links hash).

There's only one format that was using dashes (manual-section), and the
publishing tool [has now been updated to use underscores](https://github.com/alphagov/specialist-publisher/pull/342). A migration updates existing items.

All the update_types in use are currently single words.

**Note:** This should not be merged until https://github.com/alphagov/specialist-publisher/pull/342 has been deployed to prod.