WIP travel advice edition
Add country_slug validations
Added factory for TravelAdviceEdition
Add validation of state for country slug

Only allow one TravelAdviceEdition in draft or published state per country slug.
Use travel advice edition factory
Fix validation for state and country_slug
Need to require state_machine
Refactored TravelAdvice tests to use shoulda
Add uniqueness validation and index on version_number
Populate version_number on create for TravelAdviceEdition
Fix state uniqueness validation
Add build_clone method to TravelAdviceEdition
Merge remote branch 'origin/master' into add-travel-advice-edition
Remove version_number from factory as it's auto-populated
Use a context to group slug validation tests.

Also fixed tests to have better assertions.
Add travel-advice artefacts

Allow these to have a slug with a 'travel-advice/' prefix
Add published scope to TravelAdviceEdition
Add GOVSPEAK_FIELDS and SafeHtml validation to TravelAdviceEdition
Fix typo in test description
Require parted