Getting FAQ candidates requires org to be loaded which in turn requires to
load magit (via orgit) etc...
This commit moves the FAQ helm source to a new feature helm-spacemacs-faq and
removes the FAQ source from SPC h SPC so the FAQs are only available as a helm
source via SPC h f.
But we have still the FAQ org file entry in SPC h SPC so it is easy to open
the file from the main help key binding.
