AO3-6547 Extract WorkRecipients concern from Work model

[pmonfort]

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• Have you added tests for any changed functionality?
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Issue

https://otwarchive.atlassian.net/browse/AO3-6547

Purpose

Extract all work recipients logic into a WorkRecipients concern to make the code cleaner and easier to modify. The recipients code was spread across Work and CreationNotifier.

Changes:

• Create app/models/concerns/work_recipients.rb with all recipients logic moved from Work: gifts association, recipients=/recipients methods, validations (new_recipients_allow_gifts, new_recipients_have_not_blocked_gift_giver), save_new_gifts, and challenge_bypass?
• Simplify collections branch in notify_recipients
• Use keyword argument for recipients(for_form:) instead of positional boolean
• Use .exists? instead of .present? in save_new_gifts
• Use gifts.create instead of Gift.create(work: self)
• Rename challenge_bypass to challenge_bypass? (Ruby predicate convention)

No behavior changes.

Testing Instructions

Testing instructions are in the Jira issue.

Credit

Pablo Monfort (he/him)