Remove some duplication in the factories.

Also removed assigned_to from the default edition factory as it's not a
mandatory field.
Add uniqueness validation to version_number.

Also fixed tests that were setting up data in an invalid way.
Add a db level unique constraint on version_number

To catch anything the validation misses due to race conditions.
Merge pull request #158 from alphagov/version_number_validation

Add uniqueness validation to Edition version_number