AO3-4052 Require choosing a pseud option when orphaning

[pmonfort]

Pull Request Checklist

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• Have you added tests for any changed functionality?
• Have you added the Jira issue number as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-4052

Purpose

The orphan page preselected "Take my pseud off as well", so it was easy to orphan without reading the options. This PR:

• Removes the default, so neither option is checked when the page loads.
• Makes OrphansController#create reject the form when no option (or an unexpected value) is submitted, with an error and a redirect back to the orphan page.
• Reworded the options to "Remove my pseud" / "Keep my pseud", as suggested in the issue comments, and gave the group a visible prompt ("What should happen to your pseud?") plus a legend for screen readers, following the pattern used in the work form.

The partial is shared by the work, series, pseud and account orphan pages, so all four are affected.

Testing Instructions

1. Log in, open a work you created, click Edit, then "Orphan Work".
2. Neither option should be selected. The group should read "What should happen to your pseud?" with the options "Remove my pseud" and "Keep my pseud".
3. Click "Yes, I'm sure" without choosing: you should stay on the orphan page with the error "You must choose whether to remove or keep your pseud." and the work should still be yours.
4. Choose "Keep my pseud" and confirm: the work is orphaned with a copy of your pseud (<pseud> (orphan_account) in the byline, unless the work is in an anonymous collection).
5. Repeat with another work and "Remove my pseud": the byline shows orphan_account.
6. The same check applies on the series, pseud and whole-account orphan pages.

Credit

Pablo Monfort (he/him)