automatically build storybook with the new exists: directive
with gitlab 12.4 we can use https://docs.gitlab.com/ee/ci/yaml/#rulesexists
then we don't need STORYBOOK: "true" anymore
with gitlab 12.4 we can use https://docs.gitlab.com/ee/ci/yaml/#rulesexists
then we don't need STORYBOOK: "true" anymore