Reorganised domain service objects (DSOs) into separate namespace.

This follows the closing of Issue #16.

RSpec: 56 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 19 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0
Defined `have_same_blog_entries_as` custom RSpec matcher.

Used (at least initially) by our current `BlogController` spec.

RSpec: 56 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 20 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0