Added feature spec for entries in Blog index.

This replaces a model-level integration spec that Avdi introduces early
in the "getting data to stick around" section. Unfortunately, since part
of its purpose is to verify entries are in reverse-chronological order
and since timestamps only have one-second resolution, the spec verifying
post order introduces a one-second delay between adding posts.

RSpec: 137 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 47 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 3
RuboCop local overrides in our source/spec files: 3