xAPI update, asynchronous Flag, Callback function

[Yinnii]

A few new functions have been added to the social-bot-manager-service:

• xAPI URLs have been updated, and the homepage URL has been set as an environment variable for future adaptations.
• Object, verb and context have been adapted.
• An async flag is set when a service should be called asynchronously.
• The callback function will replace the old message with the new service response.
• With the GET-function, the new messages can be retrieved.