Drafts don't store correct answer

Currently, drafts cannot have a correct answer stored with them, forcing users to assign them when used. Drafts should be able to have a correct answer selected in advance.

Start by tracing how drafts are created and stored, then follow how a correct answer is assigned when a draft is used. Implement support for selecting and persisting a draft's correct answer in advance, and verify that the saved answer is available when the draft is later used.

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