Minor cleanup of #explain_errors ApplicationHelper method.

* Code now comments out ActiveModel::Errors/Mutations::ErrorAtom split.
* Spec expects Bootstrap 3 styles, along with minor cleanup as well.

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Two new ActiveInteractions, to be used by PostsController.

* PermissivePostCreator is just that; given a Blog instance and a
  `params_in` hash (with possibly omitted `title` and `body` strings),
  it creates a new Post associated with the specified Blog. If the Post
  instantiated by the call to `blog.neww_post` is invalid (because, say,
  the required :title was not specified), this interactor doesn't care;
  it hands it back to the caller as the result of successfully running
  the interactor.
* PostPublisher *does* validate the Post instance it's handed in; if the
  Post isn't valid, its errors are copied to the inter actor's error-
  reporting system (which quacks like ActiveModel::Errors). Note that
  the interactor doesn't know or care anything about what *constitutes*
  a valid Post instance; it has the model instance validate itself and
  uses its errors as appropriate.  Assuming that the Post *is* valid,
  the #execute method calls the post's #publish method and passes on any
  value that it returns.

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PostsController#create reworked to use new interactors.

This gives us nice, single-path logic in that method, which is now
*completely* ignorant of any details *at all* of what a "blog" or a
"post" *are*. This is a drop-in change that requires no other changes to
code or specs.

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PostsController#new modified to use PermissivePostCreator interactor.

This requires a small cheat in that the interactor (currently) requires
a nested hash as it would receive from submitting a form (via #create);
adding a new, private #new_post_params method to build it takes the last
bit of domain knowledge out of the #new method itself. (In a more
durable app, we'd likely choose to create yet another simple interactor
to create such a quasi-empty parameter; either that or adapt the current
PermissivePostCreator to support omitting the `params_in` parameter
entirely.)

This is another straightforward refactoring, requiring no changes to the
outside code or specs.

RSpec: 102 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 35 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5