AO3-2722 Allow parent skins to be added in wizard

[weeklies]

• Fix bug where error 500 is thrown when non-existent parent skin is added

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• Have you added tests for any changed functionality?
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Issue

https://otwarchive.atlassian.net/browse/AO3-2722

Purpose

Add a Parent Skin section to the Skin Wizard, as shown in the default editor.

Also, I've fixed a small bug where Error 500 is thrown for non-existent parent skins. I ran into this during testing and didn't find an open Jira issue, so I hope including it in here is OK.

Testing Instructions

See Jira issue.

References

There are open issues regarding Error 500s when editing skins, but this PR doesn't resolve them.

https://otwarchive.atlassian.net/browse/AO3-4820
https://otwarchive.atlassian.net/browse/AO3-3810

Credit

weeklies (she/her)

[sarken]

Also, I've fixed a small bug where Error 500 is thrown for non-existent parent skins. I ran into this during testing and didn't find an open Jira issue, so I hope including it in here is OK.

That should be fine! Do you have step-by-step instructions for reproducing it that we can add to the issue, just to make sure it gets tested?

[weeklies]

Error 500 when adding non-existent parent skin
Instructions:

• Create a site skin, with a unique name
• Under Advanced > Parent Skins, add a keysmash site skin (e. g. "keysmashwashere11")
• Submit the form.
• Please test for default and Wizard forms.

[weeklies]

If it is more convenient, I can leave a comment on Jira.

[sarken]

Thank you! And you're fine, I copied it right over and into the issue body to make sure it won't be missed.

[sarken]

I think we need some tests for these changes.

Can you add a test for the bug you've fixed in the skin parent model to spec/models/skin_parent_spec.rb?

It looks like we don't have any feature tests for adding parent skins via the interface (everything I saw at a glance looked like it relied on factories) in features/other_b/skin.feature, which I suspect might be because it's excessively difficult due to the JavaScript involved. However, even if it's not feasible to test the entire process of adding a parent skin, I think it would be a good idea to at least add a test to features/other_b/skin_wizard.feature to confirm the parent skin section is there.

[weeklies]

While testing this again, I also found that this PR also fixes another Error 500. Steps to reproduce on production:

1. Create a new site skin with a unique name (e.g. newskin101)
2. Add it (newskin101) as a parent skin
3. See an Error 500 instead of an error message shown when editing a created skin ("You can't make a skin its own parent")

This PR causes the presence validation to run and show an error message. I have changed the name of the attribute from Skin parents parent skin to Parent skin., like so: