[#47] Fix preview with versioning: QuickAnswers#preview for previews.

Adding versioning broke previews.

Previews relied on CanCan and the
article status: if the article is a draft and you are able to view
drafts, you're seeing a preview.

This approach no longer works, since there may be new draft versions of
the article which are not yet published: i.e. the most recent version is
not necessarily the one we want to show.

The solution is to add an explicit controller action for previews. In
doing so, QuickAnswersController has been refactored to reduce
duplication and keep the actions small.

CAVEATS:

Currently only working with QuickAnswers: WebServices and Guides not
supported.