Register smartdown-flows with panopticon

 - indexable_content: need to check if/how this is used. Surfacing SA artefacts based on fragments
   of nodes seems like a bad idea...
 - Registry is need of a lot of refactoring, but this minimally works
 - Not tested against a live panopticon yet
Refactor panopticon registration to explictly handle dupe flows

- Builds hashes of both sets of flows and merges, choosing Smartdown in the event of a duplicate
Print debug to stdout while registering flows to panopticon

Provides some useful debug when testing locally and extra context when looking
at jenkins logs, from when the task is run during deploy
Provide indexable_content when registering with Panopticon

For now it only uses text from the cover sheet, as i'm not convinced node text (like
title / body) is useful for indexing. After chatting with Richard Boulton he agreed.

I've tried to code this in a way that would be easy to extend to pull content from
other nodes, questions and outcomes, it should be as simple as:
 - @flow.nodes.map { |node| node_text(node) }.join
Merge pull request #1101 from alphagov/smartdown-panopticon-registration

Smartdown panopticon registration