Added update auth to PostData Pundit policy

RSpec: 683 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
PostsHelper methods now have correct URLs.

Previously, `PostsHelper#edit_post_form_attributes` had an incorrect
URL that caused routing to break (by mistakenly using the URL for
POSTing a *new* post). Helper fixed.

RSpec: 683 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected