Chat should remain open on Agent gadget even if the customer leaves the chat

*Current behavior: As soon as the customer leaves/ends a chat, it disappears from the agent interface. *

Feature to be implemented: Chat should stay on the agent interface even if the customer has ended the chat from his side. The agent should be able to still send the messages which will be saved in the database as part of the conversation history. If the agent doesn't manually end the chat after "x" minutes, a reminder/alert should be shown to the agent.

Good to have:
After "x" minutes, all those inactive chats should be shown under a new tab "Archived chats"

implement leave chat on customer gadget
implement agent-gadget disable actions other than send message
implement failover of agent-gadget (do not route chat if customer is missing in the chat)