Conversations should only store user identifier

[jdalphond-mitre]

Summary

/registry/org returns conversations with the following saved at time of write.
{author_uuid,
author_role}

It should return
{author: {username, org.shortname, role}} expanded at the time of read.

Motivation

Saving user info on write to a conversation could lead to dead links when people change orgs.

[afoote-mitre]

@david-rocca Talked to @jdalphond-mitre and this would be covered by a fix to #1715. He's okay with closing this one if you are