Gemfile and Gemfile-induced code changes.

Specifically, RuboCop-indicated cleanup, with its new SymbolProc cop.
*However*, use of that cop induces internal breakage in RuboCop, as
reported in their issue https://github.com/bbatsov/rubocop/issues/1314;
tips for mitigation/repair that would allow use of later commits in the
bbatsov/rubocop master branch would be greatly appreciated.

RSpec: 657 examples, 0 failures, 8 pending (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
Reworked CCOs.

This was originally driven by a hidden field (`:slug`) in the
post-editing form not being populated correctly, which turned out to be
an issue with the CCO. The general pain involved in how CCOs *were*
being done became readily apparent, and after far too much flail,
here's a significantly better, unified implementation scheme.

Also, in response to some Reddit traffic, became aware that the reason
I couldn't successfully bundle with any version of Ruby later than
2.0.0-p451 was because the `debugger` gem has been EOLed and its
function generally superseded by `byebug` and `pry-byebug`. Inspecting
`gemfile.lock` told me that the only thing including `debugger` was
https://github.com/rking/pry-full, which a) exists specifically as a
one-liner to pull in several `pry`-related Gems, but b) has not been
updated since 3 Feb 2013, predating Ruby 2.1. Including the individual
Gems in our Gemfile, substituting `pry-byebug` for `debugger` and
omitting https://github.com/ConradIrwin/pry-syntax-hacks which is both
(even longer-) abandoned and of questionable use.

RSpec: 662 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
Added update auth to PostData Pundit policy

RSpec: 668 examples, 0 failures, 8 pending (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
Added PostsHelper#edit_post_form_attributes method

Also extracted commonality between the two current
`*_post_form_attributes` methods into a new method. Ready to add
post-editing controller actions and views.

RSpec: 676 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected
RuboCop Issue 1314 fixed, so Gemfile use restored.

See https://github.com/bbatsov/rubocop/issues/1314 — as expected, it
took me longer to research the bug report than @bbatsov to fix the
actual bug involved. We're baaaaaack!

RSpec: 676 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected