Introduce a `Last Responding Agent` options to assign_agents action in
automations to cover the following use cases.
- Assign conversations to first responding agent : ( automation message
created at , if assignee is nil, assign last responding agent )
- Ensure conversations are not resolved with out an assignee : (
automation conversation resolved at : if assignee is nil, assign last
responding agent )
and potential other cases.
fixes: #1592
135be52431