Now using ActiveRecord for persistence of Post models.

This required several other changes to get a green bar, and suggested
several other refactorings.

* Created `Posts` database table;
* `PostsController` has a drastically streamlined `#create method`, with
  the success/failure options factored out into new, private methods;
* `PostsController` no longer has a `#build_post_for_retry` method; that
  former method's function, moved to the new `#on_create_failure`
  private method, has been moved to a new `PermissivePostBuilder` DSO;
* That new `PermissivePostBuilder` DSO *should* become the basis for a
  refactoring/cleanup of the (currently two) other DSOs that build Post
  objects;
* `Blog` model no longer has separate `AddingPosts` module;
* `Blog` model initialiser now injects method to return all entries; it
  is then used by the rewritten `#entries` method;
* `Post` model is now an `ActiveRecord::Base` subclass. Hello, Rails!;
* `Post	` model has attribute access and mass-assignment declarations
  revamped;
* `Post` model sheds methods such as `#valid?` now covered by AR;
* New `PermissivePostBuilder` Mutation-based DSO; see description in
  earlier note on `PostsController`;
* `PostDecorator` now uses a DateTime for the `pubdate` attribute rather
  than a dummy string, so ActiveRecord field-type validation accepts it;
* Extensive revamp of `Blog` model spec;
* Extensive revamp of `Post` model spec; mostly removing specs for
  features now delegated to ActiveRecord;
* WARNING: `new.slim.spec` for Post view no longer validates "inner
  value"; previously initialised to an empty string, ActiveRecord now
  sets it to `nil` if no value exists for the record field.

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