Rename existing DocumentMappings

The existing export is inconsistent and odd. It includes tens of thousands of
rows that serve no purpose, has inconsistent behaviour for different data types
and includes duplicate rows where there are multiple editions of a given item.

Changing it is therefore both risky and undesirable because it may change the
behaviour of the redirector 'munge' process, which although we are moving away
from, we need for the time being.
Basic mappings exporter
Include attachments in Mappings
Handle edition-less documents
Tighten up method name and parameters
Include work-in-progress mappings

This will allow the Transition app to provide useful information to the user
when they review a mapping for an Old URL.
Include mappings for archived content

Some archives can redirect. Maybe we could disintermediate and return
the New URL for where this redirects to?
Document some behaviours
Export localised mappings for localised sources
Make the exporter more robust
Rename some test methods, drop an unused one
Add rake task to export new-style mappings
Handle AttachmentSources without Attachments
An attachment's state depends on its visibility

There was an invalid assumption that all attachments could be considered
'published', whereas they are only accessible if the thing to which they are
attached is accessible.
Merge pull request #1155 from alphagov/newer_mappings_export

Newer mappings export