An artefact can be associated with multiple needs
Validate new need_ids must be six-digit integers

also, removing blank strings passed as need_ids,
to work around the tagsinputs ui component's
undesired behaviour.
Update need_ids when need_id is sent through API

this is mainly for backwards compatibility till
everyone updates to the latest api adapters.
Merge pull request #154 from alphagov/multiple-need-ids

Artefact can be associated with multiple need ids