Slugged PostData factory and Post entity.

Post entity has two read-only attributes, `:author_name` and `:slug`.
The slug value DOES NOT change when the title changes. DANGER: This
does not precisely match the behaviour of the underlying `PostData`
instance's `:slug` attribute.

RSpec: 412 examples, 0 failures
RuboCop: 112 files inspected, no offenses detected