NewPostForBlog refactoring

Required change in spec (obviously) and to `PostsController`, whose
`#new` method calls it.

RSpec: 127 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 46 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4
CreateAndPublishBlogPost refactoring.

Required change in spec (obviously) and to `PostsController`, whose
`#create` method calls it.

RSpec: 127 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 46 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4
PlaceholderBlogEntriesForDevelopment refactoring.

Now moved into the `Blog::PostBuilders` module, making use of the
`CreateAndPublishBlogPost` domain service object (DSO) to do just what
*it* says on the tin.

As with the earlier refactorings, there was one piece of outside code
that needed to change; rather than a controller method as before,
however, this change was in the setup for the `blog#index` view template
specs.

Also one small change to the earlier `CreateAndPublishBlogPost` DSO;
a minor bit of reformatting with no functional impact.

RSpec: 127 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 46 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4