Fix state validation on TravelAdviceEdition

Now prevents editing non-draft editions.  Allows 'save & publish'.

A side-effect of this is that it's no longer possible to directly create a published or archived edition, you have to create a draft first, then publish it etc.  I've added some factories to make this process easier in tests.
Merge pull request #51 from alphagov/fix_ta_state_validations

Fix state validation on TravelAdviceEdition