Updated new-Post view template to include image URL field.

RSpec: 126 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 43 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5
Show-Blog-entry template now calls decorator #build_body method.

A few other notes:
* Blog-index template now uses #decorate_and_render_all_entries helper
  function, rather than have an ungainly direct #render call;
* That helper function is a wrapper around a new `DecoratedPostRenderer`
  class that has a simple command-pattern interface with sensible
  dependency injection (in case we ever need to change the decorator
  class or partial name used, or even reuse the class for other partials
  later);
* Since the helper function and supporting class were extracted from a
  fully-tested template (which still uses them), they don't (yet?) have
  their own standalone specs. Oh well.

RSpec: 127 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 43 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5