AO3-7506 Add backfill task for missing username pseuds

[pmonfort]

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Issue

https://otwarchive.atlassian.net/browse/AO3-7506

Purpose

Adds an After:backfill_missing_pseuds rake task that finds users who don't have a pseud matching their current username and creates one.

This covers the ~100 users who deleted their username-matching pseud before the fix in AO3-6359 (PR #5534) was deployed.

One difference from the SQL in the ticket: I added AND pseuds.user_id = users.id to the subquery. Without it, if another user happens to have a pseud with the same name as the affected user's login, the query would miss that user.

Testing Instructions

Testing instructions are in the Jira issue.

References

• AO3-6359 / AO3-6359 Fix Possible to delete the pseud that corresponds with your username if the capitalization or diacritics differ #5534 — the fix that prevents deleting the username-matching pseud

Credit

Pablo Monfort (he/him)