`WikiPage` should have a slug even when not persisted.
1. So we can build the markdown preview URL for it. 2. We can't skip the slug in this case, because the slug is used to construct relative markdown URLs. 3. Add rspec feature tests to cover creating wiki pages with spaces/hyphens in the name. 4. Add rspec feature tests for markdown preview URL rewriting, which was only covered by unit tests up to this point.