Disable the deploy button if there are no changes.
Merge pull request #383 from huwshimi/deploy-button-disabling

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