Allow the artefact's need_id to be changed when < 100000

Need IDs below 100,000 are Needotron need IDs. We'd 
like to be able to change these to use Maslow need 
IDs. 

This changes the Artefact's need_id_editable? criteria 
to allow the need_id to be changed when it is less 
than 100,000.
Tidy up the artefact enhancements class

- Extract the Maslow Need ID lower bound into a constant
- Change the need_id_editable? method to compare against
  need_owning_service
- Add a new method to check if the need id is numeric or not
Merge pull request #101 from alphagov/allow-editing-of-needotron-ids

Allow the artefact's need_id to be changed when < 100000