Encourage users to reuse queues

[nwalters512]

Many people are deleting queues at the end of the day or are creating new queues when a closed queue already exists. We should add features that push people towards the close/reopen flow.

• In the "create queue flow", we should surface a list of existing open/closed queues and prompt them to reuse them.
• Add a "would you like to close instead?" option to the "delete queue" flow.
• Prompt to close the queue when the last staff member leaves.