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function generally superseded by `byebug` and `pry-byebug`.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: 680 examples, 0 failures (but see Issue #87)
RuboCop: 151 files inspected, no offenses detected