AO3-6789 Return 404 when accessing challenge on nonexistent collection

[gemmie]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6789

Purpose

Return 404 when trying to signup to a challenge on nonexistent collection

Testing Instructions

1. Log in

2. Try to go to the sign-up page for a challenge in a nonexistent collection, e.g. https://test.archiveofourown.org/collections/FakeCollectionPlaceholderName/signups/new

3. 404 should be returned

Credit

JIRA name: gemmie