BlogDataDecorator discombobulated.

As BlogData (the implementation model) is on its way out,
removing/moving dependencies and interactions. Here, the shifted code
was the former BlogDataDecorator#summarise method, moved (at least
temporarily) to BlogHelper#summarise_blog.

More to come.

RSpec: 691 examples, 0 failures (but see Issue #87)
RuboCop: 154 files inspected, no offenses detected