Cleanup; we need Post AND` User entity verifiers.

Calling the entity-verifier matcher for Posts simply
`be_saved_entity_for` was a mistake.

Also started breaking shared examples out of `user_repository_spec`
that will be used for the (imminent) `post_repository_spec` as well.

RSpec: 883 examples, 0 failures (but see Issue #87)
RuboCop: 176 files inspected, no offenses detected