`SelectionStateModel`'s methods `isSystemDrive` and `isDriveLocked`
don't depend on application state. They have been extracted in a different
AngularJS service: `DriveConstraintsModel`. The new service's actual
implementation is in `lib/src`, in order to be reused by the CLI.
Miscellaneous changes:
- Rename `lib/src` to `lib/shared`
- Refactor `drive-constraints` to throw when image is undefined
The default behaviour was to pretend that we're all good if the if
the image is not specified. We're not using this "feature", and
it can be dangerous if we forget to pass in the image.
- Make `isSystemDrive` return `false` if `system` property is undefined
- Use `drive-constraints` in `store.js`
Change-Type: patch
