Core: Fix symlinks in static dir when building static Storybook

[ghengeveld]

Issue: -

What I did

• Fixed symlink expansion in static dir when building Storybook
• Keep original file timestamps when copying static dir

How to test

• Is this testable with Jest or Chromatic screenshots? no
• Does this need a new example in the kitchen sink apps? no
• Does this need an update to the documentation? no

If your answer is yes to any of these, please make sure to include it in your PR.

[shilman]

LGTM!