Improve nuxeo-tags API with auto type

In nuxeo-document-task we had the type set to user,] and this could lead to incorrect tags if we had both users and groups as assigned users (same for delegated actors). 

In WEBUI-308, to avoid updating (or breaking) the API, we suggested split that into two separate nuxeo-tags groups, even thought it doesn't solve the issue in loaded layouts.

We could improve the nuxeo-tags API extending the type property with auto or user/group and move the logic of generating the correct tags into the element. Notice that this will require an update on the layouts as well.