[Feature] Added Chatbot

[Hendremy]

Following https://github.com/frappe/chat/issues/65

I added a default Chatbot user with whom it is possible to send messages to directly via a chat room. It can be disabled through the Chatbot Settings.

When a user sends a message in a room where the Chatbot is a member, the "Action" method will be called providing the sender email, the room id and the message content. It then returns a message that will be sent to the room. This "Action" has to be configured by the admin and can be linked to any Chatbot.

When a user clicks on the "Chatbot" button, it will create a Direct room to the Chatbot and then open that room.