Killed :policy_scoped on BlogController.

That was the easy part, as expected.

Moving the #summarise method from BlogDataDecorator (which no longer
applies unless we assign `@blog`) to, e.g., `BlogHelper` wasn't the fix
we were looking for. The rethink continues.

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