Second whack at PostsController#edit.

Now does not assign variable unless authorisation succeeds. Specs cover
all three authorisation scenarios (author, guest, non-author user).

Routing updated to allow all actions for posts *except* :destroy (we
never delete posts) and :index (the Blog#index method/action does this,
at least now).

RSpec: 685 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected
Broke partials out of post/show template.

Working our way back to the post/edit template. Honest.

RSpec: 685 examples, 0 failures (but see Issue #87)
RuboCop: 148 files inspected, no offenses detected