Disable the deploy button if there are no changes.

The deploy button should be disabled when there is nothing to deploy.

QA:
- load the gui with the mv flag
- check that the deploy button is grey and that clicking on it does nothing
- drag a charm to the canvas
- the deploy button should be orange and clicking on it should open the summary panel
Instead of doing this check in multiple places in the codebase what about a single method that takes a boolean value which handles enabling or disabling the button?

Something like...
```
this._toggleDeployButtonStatus(changes > 0);

...

_toggleDeployButtonStatus: function(enabled) {
  if (enabled) {
    // remove class
  } else {
    // add class
  }
}
```