Do not require stack selection for new guides
Do not require stack selection for new guides

- We only have a finite set of stacks and it's possible a guide doesn't fit
  neatly into any of them.  So, allow empty stack to go to the catch-all
  'other' stack.
- Related to #69

paulocheque/guides-cms,pluralsight/guides-cms,paulocheque/guides-cms,pluralsight/guides-cms,paulocheque/guides-cms,pluralsight/guides-cms