start adding feature specs for blog post preview

~~WIP~~ This is for  #246
also closes: #259 #253 #252 #249
Can be written shorter, and more ruby-ish. Shorter is not necessarily always the best thing, but more ruby-ish is, most of the time.

```ruby
@post.draft = params[:commit] != 'Publish'
```

however, DRY is also ruby-ish, and since the same code appears again below, what about putting it into a private method?

```ruby
@post.draft = post_params_draft
...
private
def post_params_draft
  params[:commit] != 'Publish'
end
```

or even better, maybe it can be incorporated into post_params (not sure if that works, I hope params behaves like a hash):

```ruby
  def post_params
    params
      .require(:post).permit(:title, :description, :picture, :full_name)
      .merge(draft: params[:commit] != 'Publish')
  end
```