The presence of Storybook’s preview-head.html file in the repository
is a constant annoyance: it’s only needed for Storybook and it changes
all the time, producing a lot of Git noise. By making it a separate
step to have the Ember CLI server running before starting Storybook,
we no longer need to have preview-head in the repository. It needed to
be present because there was a race condition where it was sometimes
not generated in time for the Storybook parallel startup.