As an admin, when I change work slugs I want past slugs stored and used for redirects on public views so that links for edited works continue to resolve.

[rlskoeser]

testing notes

The scenario for this is if someone cites or links to a book page (including urls in the data exports) and we change records in the database, we want that link to still resolve if possible. The easiest way to test it might be this: choose a work record on the test database and open up multiple different work detail pages in different tabs (bibliography, circulation activities, card list) so that you have URLs ready to test after you make changes. Then test that:

• when you edit a work and change the slug, the a past slug — it should be visible but read only in the admin edit page
• all work details pages for the old slug should redirect to the new slug — you should see the content instead of a 404, and the location in your browser should show the new slug
• test that if you add or edit a work and try to use a slug that is a previously used slug on a different work, you get a validation error

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Should be able to adapt implementation from #434 (but simpler since there is no book merge method).

dev notes

• add alternate/old slugs field to work
• make old slugs visible (read only) in admin
• validate new slugs to avoid collisions with old slugs
• add old slug on save if slug changes
• permanent redirect on old slug match for all work detail pages

[rlskoeser]

@jkotin this is available for testing now, see the notes in the issue description. It's similar to functionality we previously implemented for members that Cate tested and signed off. I decided it was important to implement now to make the book urls in the data exports more durable.