ci: optimize unicorn deployments

[janivo]

Currently, you have to trigger the unicorn deployments manually. It would be great if that happens automatically. To prevent unnecessary runs and to save resources, the following should be considered:

• The pipeline should only trigger on elements or storybook package changes (e.g. check the scope of the pull request title?)
• The pipeline should only run if build/test was successful
• We should drop the build/test step because it's guaranteed to run beforehand. Instead, we should download the elements and storybook artifact from the previous step
• Outdated unicorns should be destroyed automatically