MIE-1524 resolved the first time an activity is generated it is shows duplicated

Resolved bug "the first time an activity is generated it is shows duplicated". 

The problem was due we were listening to a deferred object created in ActivityModel to add the model to the Backbone collection. By doing this, we are not taking the benefits of using Backbone collections, which for example, won't add a new model to the collection if the model existed previously. 

So removed the bindings in *addActivity*, which should only add a model to the collection, and instead we added that binding in the *appendActivity* which adds the element to the DOM.