public IHttpActionResult PostActivity(PostActivityModel model)
{
var activityIdBytes = Encoding.UTF8.GetBytes(model.ActivityId);
var streamIdBytes = Encoding.UTF8.GetBytes(model.StreamId);
var activity = new Activity(streamIdBytes, activityIdBytes, model.Body, string.Empty);
WebApiApplication.ActivityRepository.Append(activity);
return Ok(model.ActivityId);
}