AD tightening

Changed the field in Zendesk that stored the phone number from a
strictly numeric field to a string field. The reason for that is so that
the validation of the input only happens once, in the Rails app, rather
than twice in the Rails app & Zendesk.

Also tightened the validation of the phone number as it was allowing
alphabetic characters.
Added error messages to the view

Spoke to Leigh about the format of the messages before adding them.
Merge pull request #397 from ministryofjustice/zendesk-validation

AD tightening