Failing tests to edit explanation for archiving
Form to update explanation for archiving

https://www.pivotaltracker.com/story/show/70353932

Managing editors should be able to edit the
explanation they provided when they archived
the edition. Most realise they made typos, or
need to make corrections once they see archived
editions on GOV.UK and request changes through
zendesk. This allows them to do it on their own.
Merge pull request #1466 from alphagov/edit-archiving-public-explanation

Edit archiving public explanation