AO3-6714 Prevent tag sets becoming ownerless

[tommilligan]

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• Have you added tests for any changed functionality?
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  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/AO3-6714

Purpose

This PR implements point 2. of the "What should happen" section of AO3-6714. For that specific case, if you:

• Are already an owner of a tagset more than once
• Edit the tagset
• Remove yourself as an owner

You are now only removed as a tagset owner once. You remain a tagset owner at least once. This means the tagset cannot be orphaned.

In more general terms, this PR enforces that when removing owners from a tagset:

• The number of TagSetOwnership records removed is at most one
• At least one TagSetOwnership records exist for an OwnedTagSet

Testing Instructions

Follow the reproduction instructions in AO3-6714. You will still be able to create a tagset where a single user owns it twice, but you can no longer remove all owners from the tagset.

References

N/A

Credit

What name and pronouns should we use to credit you in the Archive of Our Own's Release Notes?

Tom Milligan (they/them)

[brianjaustin]

Thanks for the PR @tommilligan! I saw that you also have #4864 open for this issue, and since that one seems to be more complete, I'm going to close this one. (In general, we tend to stick with 1 PR per issue because otherwise there can be some unfortunate merge conflicts that are annoying for folks to untangle.)