Improve assignment queue visibility and control for admins

Problem
When conversations queue due to capacity limits in load-balancing rules, admins have limited visibility and control—they can't see the hidden queue or prioritize which conversations should be assigned first.

Request
Give admins visibility into the assignment queue (conversations waiting to be assigned), the ability to view queue status and manually prioritize conversations between multiple queues, and the option to create conversation queues without using load-balancing rules.