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.
None, really. I should extend from there.