Show abstract, details, pitch diff in proposal

[barrettclark]

I've iterated a little on the proposal diff idea. I am trying to do the simplest possible implementation and give us foundation to build from. The components of the diff viewer are:

• PaperTrail gem to save versions of the proposal as it is updated
• Diffy gem to present a customary diff view of proposal changesets

If there are updates to the abstract, details, or pitch fields the diff viewer will be displayed:

I would love feedback, and even help fine-tuning this and getting it in place.

This resolves https://github.com/rubycentral/cfp-app/issues/23