Singleton Blog instance now shared among all controllers.

* Initialised shared blog instance in `config/initializers/blog.rb`;
* Also added `#reset` action to BlogController until we get a real
  database that can be setup for test purposes using, e.g., the
  DatabaseCleaner Gem;
* Blog controller spec calls controller's `#init_blog` method (to ensure
  that the shared instance is set) and `#reset` (to clear blog-entry
  collection) before checks for assigned blog instance attributes;
* Reworked `have_same_blog_entries_as` custom matcher for more
  intelligible error reporting;
* Introduced our first Rubocop local override in ApplicationController;
  see commentary there.

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: 1