AO3-5539 Fix hidden tags functionality for prompts/requests/external works

[techlingyi]

Pull Request Checklist

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• Have you read the contributing guidelines?
• [partially] Have you added tests for any changed functionality?
• Have you added the Jira issue number
  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/AO3-5539

Purpose

Makes it possible for users who have set their preferences to have warnings or additional tags hidden by default to click the "Show Warnings"/"Show Additional Tags" button to show warnings/additional tags on prompts (for prompt memes), requests (for gift exchanges), and external works.

Testing

Locate/create a prompt meme that allows prompts with warnings and additional tags; locate/create a prompt that has warnings and additional tags.

Locate/create a gift exchange that allows requests with warnings and additional tags; locate/create a request that has warnings and additional tags.

As the Jira issue says, log in as an admin, locate a bookmark of an external work, and use the "Edit External Work" option to add additional tags to it.

Then sign in as a different user from any of the above, go to the preferences page and set your preferences to hide warnings/additional tags by default, then go to the prompt, request, and external work bookmark above to check if the Show Warnings/Show Additional Tags buttons work.

Notes

I added a Cucumber test for showing hidden additional tags in prompts, but found it difficult to understand the infrastructure of the gift exchange and prompt meme step definitions, so couldn't figure out how to add Cucumber tests for anything else. It's fairly easy to test manually, but let me know if I should try again.

Credit

Ling-Yi (any pronouns)

[techlingyi]

It looks like Requests can't have their warnings/freeforms accessed with .archive_warnings or .freeforms, maybe because they're not Taggables? So I decided to just make all objects (except Series) access warnings/freeforms with tag_groups, since it also works for the other types of objects. Let me know if there's performance concerns with this, though!