Remove duplication of policies in the speed tagging UI

The view code still leaves open the possibility of duplication in the UI of role appointments or ministerial roles.  I'm not sure, but I think that's a presentational thing where we show someone's name twice, but it's because of them having multiple roles.  Would like to see someone report an issue before fixing it.
Make document_sources factory useful

Use double quotes so the url is interpolated and we actually get a benefit from it being in a sequence.
EdtionsWorkflowController#convert_to_draft sends you to the import list
Better duplication removal for policies while speed-tagging

Make sure we use only the latest edition of a policy rather than showing all editions.
Merge pull request #130 from alphagov/speed-tagging-fixes

Speed tagging fixes