Fixed #15190 -- Refactored the collectstatic command to improve the symlink...

Fixed #15190 -- Refactored the collectstatic command to improve the symlink mode and generally straighten out its behavior.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15388 bcc190cf-cafb-0310-a4f2-bffc1f526a37