Initial CreateAndPublishBlogPost mutator.

Only knows about posts with :body attributes; no image URL yet.

* PostsController replaces explicit post creation and adding to blog in
  #create method with call to CreateAndPublishBlogPost.run!;
* Blog model no longer adds PlaceholderBlogEntriesForDevelopment;
* New feature (UI-level) spec to create and publish blog post;
* New "user visits landing page" feature spec.

A note on the naming conventions used in the feature spec: a "member" is
a logged-in user (once we have logged-in users). A "user" is a user,
regardless of login status.

Note that when we add image posts (real soon now), the model, the view,
and the business domain object (in this case, a Mutations::Command) will
change but the controller (and controller spec) shouldn't need to.
(There will also be new feature specs, of course).

RSpec: 103 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 33 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 3
RuboCop local overrides in our source/spec files: 3