Remove the unplaced unit when it has been placed.
Merge pull request #360 from huwshimi/remove-placed-token

This branch removes the unplaced unit token once it has been placed. A followup branch will create the new token once placeUnit has been fixed.

I had to fix up a bunch of tests so that the removal worked in those tests.

QA:
- Drag a service to the canvas.
- Open the machine view.
- Drag and drop the unplaced unit on 'Create new machine'.
- The unplaced unit token should disappear.