[5] Bot Session Timeout warnings

[chadgoss]

see RocketChat/Apps.Dialogflow#21

Allow a configurable timeout warning for user-bot conversations

Similar to the agent timeout but based on a configurable value set on the rocket.chat server. Could also be based on the Dialogflow timeout value, but currently we just set this value manually.

--> An event should be sent to the NLP when this timeout is reached so we can send a message to the user saying that their session has timed out before ending the chat.

--> send end_session dialogflow event when chats timeout