Completed the 'Submitting Posts' chapter.

This gets us through Page 45 in the e-book/p-book, and [the last coding page in the chapter](http://objectsonrails.com/#ID-17445728-2450-47ad-b6da-eb2194b503d3) in the online version.

Highlights:

- New `PostsController` supports #new and #create actions, using (new) `NewPostForBlog` and (new) `CreateAndPublishBlogPost` mutators, respectively;
- `ApplicationController` now defines a `@blog` object shared among all controllers, defined in the new `config/initializers/blog.rb` file. (It also silences a RuboCop complaint that we're not using Rails 4 terms);
- The `BlogController` no longer uses the `PlaceholderBlogEntriesForDevelopment` Mutator to load dummy data, and uses the shared `@blog` object defined by `ApplicationController`;
- The `Post` "model" class uses a couple of ActiveModel modules to play nice with `form_for`. It also now supports setting the body element from the constructor;
- New "New Post" form view;
- Routes for the Posts controller have been added;
- Bootstrap styling added to the `application.css` file and Scripts to the `application.js` file;
- Rails flash support very hackishly added to the application layout (and marked as a FIXME);
- The main Rails application config has been updated to silence a warning in newer versions of the i18n Gem used by Rails;

And we still have complete test coverage!

RSpec: 74 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 27 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop local overrides in source and own specs: 1