Likes feature

**Backend**: (@kartones):
- Refactor of get_authenticated_users
- Adds 'current_viewer' to query for authenticated users 

**Frontend** (@javierarce)
- Creates a public model and view to manage the likes in the public dashboard and embeds.
- Attaches a submodel (Like) to the visualizations to manage the likes in CartoDB Editor.
This class is not sending warnings through Rollbar, so we won't realize, isn't it (I don't know whether 403's are automatically logged).