Feature/add classNames for relative position to active slide

[strindhaug]

Implementing the feature described in https://github.com/brainhubeu/react-carousel/issues/670

Adding

• If slide is before the active it has the className BrainhubCarouselItem--before
• If slide is after the active it has the className BrainhubCarouselItem--after
• All slides has the relative index as a className BrainhubCarouselItem--relindex-X (where X is the absolute difference between its index and the selected index)

[strindhaug]

I added two simple tests, thinking the build failed because there was no test coverage or something... But now I see that its some other errors that seems unrelated to my PR that is the problem.

No idea how I'm supposed to fix this error in "test":

The DANGER_GITHUB_API_TOKEN/DANGER_BITBUCKETSERVER_HOST/DANGER_GITLAB_API_TOKEN environmental variable is missing Without an api token, danger will be unable to comment on a PR

And the missing password in "deploy_test_environment".

Seems like there is some configuration errors.