Make unplaced units drag and droppable on container elements

This branch makes the unplaced unit token draggable and droppable on the container token and container header. 

#### To QA
- Use the il and mv flags
- Run 
```
ecs = app.env.get('ecs');
cb = function() { console.log(arguments); };
app.env.addMachines([{}], cb);
ecs.commit(app.env)
```
- You should now have a machine in the machine column
- Drag a service to the canvas and switch back to machine view
- You should now have an unplaced unit
- Select the machine (It should now show it's constraints)
- Drag the unplaced unit token onto the 'New Container' header drop target
- Click Deploy in the deployer bar
- Click Confirm in the deployer bar
- Click the machine, a container should now be listed in the third column