Rethinking; wandering (farther) from the book.

Simply put, Avdi's dichotomy between 'image' and 'text' Post instances (while maintaining single model and controller classes and unified views), while clearly intended as a demonstration of his [`Exhibit` pattern](http://objectsonrails.com/#sec-12_1), is Bad Design(!) that has had me chasing my own tail for far, far too long.  *Our* Post instances, henceforth, must have either a non-empty body or a non-empty image URL, *or both*. We'll still be able to display "everything" using a single call from the view (no matter what the actual instance content), which was the final goal of that section of the book anyway.

This will drive one or more immediately subsequent commits cleaning up the flailing done in support of the differences within a single concept. Pfffft.

RSpec: 113 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
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RuboCop global overrides in `.rubocop.yml`: 4
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