Users other than author cannot edit existing post.

The Guest User and any registered user other than the post author are
prevented from editing a published post, either by directly navigating
to the "edit" page (which has been blocked since Pundit edit-action
policies were implemented), or by clicking an "Edit <article-name>"
button (which is only shown to the author).

RSpec: 702 examples, 0 failures (but see Issue #87)
RuboCop: 150 files inspected, no offenses detected