[FEATURE] Agent Session Timeout warnings

[dlassalle]

• Allow a configurable timeout warning for user-agent conversations
  • Inform customer with a countdown modal when their conversation is about to timeout.
  • This timeout is based on a Salesforce configuration that needs to be understood/respected by the Rocket.Chat Client.
• When timeout is reached, salesforce will end the session

[PrajvalRaval]

Won't be possible since methods like setTimeout or setInterval is not supported by Apps Engine -> RocketChat/Rocket.Chat.Apps-engine#278