Add a service unit when dragging the service.
Merge pull request #309 from frankban/place-unit

Also improved the changeset summary page + clean up.

QA:
- make devel;
- visit `/:flags:/mv/il/`;
- disable the simulator;
- drag a service;
- hit deploy in the status bar;
- you should see the deployment summary: if the description
  hides, click cancel and then deploy again (there is a XXX comment
  about this issue);
- click confirm, the service should be deployed with its unit;
- in the JS console `app.db.units.size()` should return 1 and
  `app.db.units.item(0)` should show the unit with the expected
  properties;
- the machine view should correctly show the unit (on machine 0).

TODO:
- do not deploy unplaced units;
- env.placeUnit to place a unit.