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. A migration
updates existing items.

All the update_types in use are currently single words.
Merge pull request #88 from alphagov/underscored_formats

Don't allow dashes in format or update_type