More on replacement Post CCO and specs.

Post implementation-to-entity conversion now handles slug properly. The
main effort this commit went to creating a group of shared examples for
the PostCCO#to_entity method that let us concisely describe the salient
expectations in each context.

RSpec: 763 examples, 0 failures (but see Issue #87)
RuboCop: 153 files inspected, no offenses detected