Allow .json stories to be generated without bootstraping Drupal in CICD environment

Problem/Motivation

Since *story.json files are generated files, we do not want to commit them to the repo but instead, while running our CICD process and building a site for deployment, we would like to generate .json stories. However, `storybook:generate-all-stories` requires a full Drupal environment. (We do not want to require constructing a full Drupal environment for each code change just to generate stories.)

Proposed resolution

Make the `storybook:generate-all-stories` drush command available globally and remove full-Drupal-environment dependencies from that command so that we could call `vendor/bin/drush storybook:generate-all-stories` directly during our build process.