AO3-5657 Reassign tag set nominations when pseud is deleted

[pmonfort]

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Issue

https://otwarchive.atlassian.net/browse/AO3-5657

Purpose

When a user deletes a pseud, tag set nominations associated with that pseud are left orphaned in the database. The owned_by scope uses an inner join through the pseud, so these orphaned nominations become invisible to the user.

This PR adds tag set nomination reassignment to Pseud#replace_me_with_default, following the same pattern used for comments, gifts, challenge signups, and collection participants. If the default pseud already has a nomination for the same tag set, the conflicting nomination from the deleted pseud is destroyed (along with its associated tag nominations via dependent: :destroy). Otherwise, the nomination is reassigned to the default pseud.

Testing Instructions

1. Log in as User A.
2. Go to Browse > Tags > Tag Sets > New Tag Set.
3. Check the "Currently taking nominations?" box and set at least one nomination limit to a non-zero value.
4. Fill in the rest of the fields and press "Submit" to create the tag set.
5. Log in as User B.
6. Go to My Preferences > Manage My Pseuds > New Pseud and create a new pseud.
7. Fill in the fields as desired and press "Create" to create a new post.
8. View the tag set created by User A and click "Nominate."
9. Select the pseud that you just created.
10. Nominate tags as desired, and then press "Submit."
11. Go to My Preferences > Manage My Pseuds.
12. Find the pseud that you just created/nominated for, and press the "Delete" button.
13. View the tag set again and click "Nominate."
14. Expected: You should be redirected to your existing nominations (now under your default pseud), not a new nomination form.

Credit

Pablo Monfort (he/him)