More Markdown-related Post body changes.

* New post helper for feature specs now generates a post body with an
embedded inline element (emphasised text); this helps us prove that we
can in fact handle embedded Markdown;
* New "Member can publish an article and view it on the landing page"
spec proves that that "embedded inline element" is correctly rendered;
* Minor cleanup to PostDataDecorator spec;
* PostDataDecorator's ImageBodyBuilder spec strips a
meaningless-in-context newline that's always added to generated content
by RedCarpet;
* PostDataDecorator's ImageBodyBuilder code converts the passed-in body
markup to HTML (from Markdown/HTML/who cares?);
* New PostDataDecorator::SupportClasses::ContentConverter class to do
that conversion;
* Ripping the Markdown-aware stuff out of the main PostDataDecorator
class, now that we've the shiny new ContentConverter support class.

RSpec: 498 examples, 0 failures
RuboCop: 129 files inspected, no offenses detected