Add unarchiving task

https://www.agileplannerapp.com/boards/173808/cards/8018

Provides a simple way to 'unarchive' Editions by creating a draft version and force publishing it.
See discussion here https://www.agileplannerapp.com/boards/173808/discussions/2428 for more background.
This shouldn't be necessary (and is also undesirable) as an archived edition should already have publishing timestamps which the new draft inherits. I think the problem is that our factories create a really bad archived edition, e.g. `FactoryGirl.create(:edition, state: "archived")` creates an edition in an "archived" state minus the usual publishing timestamps and an `Unpublishing`! For the purposes of this test, you could create a published edition (with `create(:published_edition)`) and then archive it with `@edition.archive!`. This will be closer to a real archived edition and you'll be able to remove this line.