Organize callouts code under users

What does this MR do and why?

• organizes the callout code under users/ in the model, controller and helper layers.
• defaults user_callout to be the default callout and drops the prepend user on those items as it is now nested under users

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #340653 (closed)