FeatureSpecNewPostHelper updated.

`FeatureSpecNewPostHelper#fill_in_new_post_form` no longer forces all
articles to be `public`, but honours the setting passed in from the
initialiser's `data` parameter and/or the value passed to `#step`.

RSpec: 804 examples, 0 failures (but see Issue #87)
RuboCop: 154 files inspected, no offenses detected