Maintain redirects to canonical identifiers for objects

[cokernel]

Each object in ExploreUK should have a single identifier. However, sometimes we accidentally create more than one identifier for the same object. In these cases, ExploreUK should have a mechanism to mark the duplication and designate a specific identifier as the canonical identifier. A content manager or admin should have a way to use this mechanism, and also should be able to select which copy of the object should be served to users.

I think the "As a <role>" wording is not valid Gherkin syntax, but I've included it to separate content manager scenarios from visitor scenarios.

Feature: Objects have canonical identifiers
    As a content manager

    Scenario: Most canonical identifiers are set automatically
        Given the Rupert file does not exist in ExploreUK
        When I add the Rupert file to ExploreUK with identifier ABC
        Then the Rupert file should have canonical identifier ABC

    Scenario: Select a canonical identifier when duplicates exist
        Given the Rupert file has canonical identifier ABC
            And a copy of the Rupert file has been added to ExploreUK with identifier XYZ
        When I instruct ExploreUK that XYZ is a copy of ABC
        Then XYZ should become a non-canonical identifier of the Rupert file
            And ABC should remain the canonical identifier of the Rupert file

    Scenario: Select a canonical object when duplicates exist
        Given the Rupert file has canonical identifier ABC
            And a copy of the Rupert file has been added to ExploreUK with identifier XYZ
        When I instruct ExploreUK that the canonical content for the Rupert file is currently at XYZ
        Then the canonical copy of the Rupert file should use the content currently at XYZ


    As a visitor

    Scenario: User visits item through canonical identifier
        Given the Rupert file has canonical identifier ABC
        When I visit the ExploreUK item at identifier ABC
        Then I should access the Rupert file

    Scenario: User visits item through non-canonical identifier
        Given the Rupert file has canonical identifier ABC
            And the Rupert file has non-canonical identifier XYZ
        When I visit the ExploreUK item at identifier XYZ
        Then I should be redirected to the ExploreUK item at identifier ABC