Add more context to audit log results when sp user agent is reassigned

[bronwyncombs]

Is your feature request related to a problem? Please describe.
From Dan @ CSIRO:

We have some triggers based on the Specify audit tables to help us synchronise between our herbarium collection in Specify and the Living Collection stored in another database. However we noticed that when assigning a new user to an existing agent, or re-assigning the agent associated with a user that the change to the SpecifyUserID field in the agent table is not captured in spAuditLog or spAuditLogFields.

This action updates the Agent ID associated with the SpecifyUserID.
In the audit log:

• Action is reflected by the record version
• Table Num column shows Specify User and Record ID has the user's name.
• Old Value and New Value fields are blank

Describe the solution you'd like

We were expecting that since the SpecifyUserID field is in the agent table that we would get a row in the auditlog tables - for an initial assignment we want to see Old Value = null and new Value = the assigned user, and for updates similarly we want to see the old and new SpecifyUserID value.
It doesn't make sense to us that updates to other fields in the agent table are captured in the audit log but this one is not.

Reported By
CSIRO