AO3-6857 Improve prompt meme anon tickbox behavior

[techlingyi]

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• Have you added tests for any changed functionality?
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Issue

https://otwarchive.atlassian.net/browse/AO3-6857

Purpose

Previously, for a prompt meme with default-anonymous prompts, all your previously added prompts would default back to a checked anonymous tickbox when you edited your signup, even if you unchecked the tickbox when first creating them. This PR makes it so previously added prompts have their original anonymity status preserved.

Testing Instructions

Anonymous by Default:

1. Create or find a Prompt Meme with "Prompts Anonymous by Default" checked (https://test.archiveofourown.org/collections/testing_assignments_edited is available on Staging)
2. Create a prompt sign-up - deselect "Semi-anonymous prompt?", which should be selected by default - and save
3. Press "Edit Sign-up"
4. Verify that the first prompt still has the "Semi-anonymous prompt?" unchecked
5. Add another prompt, this time leaving "Semi-anonymous prompt?" still selected, and save
6. Press "Edit Sign-up"
7. Verify the first prompt's tickbox is unchecked and the second prompt's tickbox is checked
8. Add a third prompt and deselect "Semi-anonymous prompt?", and enter some noncanonical tag before saving
9. The prompt form should error out
10. Verify that the first prompt's tickbox is still unchecked, the second is still checked, and the third is also checked (unfortunately) (a sacrifice for the sake of not overcomplicating it)

Not Anonymous by Default:

1. Create or find a Prompt Meme with "Prompts Anonymous by Default" not checked
2. Create a prompt sign-up - leave "Semi-anonymous prompt?" unchecked, which it should be by default - and save
3. Press "Edit Sign-up"
4. Verify that the first prompt still has the "Semi-anonymous prompt?" unchecked
5. Add another prompt, this time selecting "Semi-anonymous prompt?", and save
6. Press "Edit Sign-up"
7. Verify the first prompt's tickbox is unchecked and the second prompt's tickbox is checked
8. Add a third prompt and select "Semi-anonymous prompt?", and enter some noncanonical tag before saving
9. The prompt form should error out
10. Verify that the first prompt's tickbox is still unchecked, the second is still checked, and the third is also checked (yay!)

References

Undoing some stuff from https://otwarchive.atlassian.net/browse/AO3-2614, corresponding to some of the most recent commits on the prompt form view

Credit

Ling-Yi (any pronouns)