App root now 'posts#index'.

The `posts#index` view now renders a page identical to the
still-existing `blog#index` view.

Now to rip out the Blog stuff.

RSpec: 866 examples, 0 failures (but see Issue #87)
RuboCop: 164 files inspected, no offenses detected
[Closes #109] Ripped out BlogData-related code.

The `Blog` domain entity still lives, but the controller, model (and
underlying database table), helpers, views and corresponding specs are
gone.

RSpec: 810 examples, 0 failures (but see Issue #87)
RuboCop: 152 files inspected, no offenses detected