Mentions require a a DSNP User URI instead of an Id

[wilwade]

Problem

Mentions were requiring a DSNP User Id instead of a URI as is specified link to Pivotal Tracker #179340132

Solution

Required a DSNP User URI for the validator and the factory

Double Checks:

• [x] Did you update the changelog?
• [x] Any new modules need to be exported?
• [x] Are new methods in the right module?
• [x] Are new methods/modules in core or not (i.e. porcelain or plumbing)?
• [x] Do you have good documentation on exported methods?