Use display name in user tagging

User tagging (COLLAB-3321) is a great addition, but when it displays tagged users as their login name, it's hard to use with SSO.  With my company's SSO service, the username value is a long, cryptic string that uniquely identifies users, but users never see this name in normal use of company systems.  Our users update their display names to show their actual names and it would be helpful to show this value instead of the SSO ID assigned to the login field.