AO3-5100 List all Open Challenges with pagination and filtering

[ayasayadi1]

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Issue

https://otwarchive.atlassian.net/browse/AO3-5100

Purpose

Merge Gift Exchange and Prompt Meme lists

list_challenges runs two separate queries (one for Gift Exchanges and one for Prompt Memes), concatenates and caps the results at 15. It now runs a single query across both challenge types, paginated and sorted by closing date.

Include challenges with no closing date

The open challenges query filters by signups_close_at > now, which silently excludes challenges with no closing date. The filter now matches challenges closing in the future or challenges with no closing date set.

Filter Open Challenges

Open Challenges can now be filtered by title, tag, multifandom, closed, moderated, and collection type (the "No Challenge" option is hidden since it's all challenges)

Gift Exchange Challenges and Prompt Meme Challenges support the same filters, except collection type.

Credit

Aya Sayadi