Document-backed writing prompts

Right now, prompts are solely tied to empty fields on content pages. For more complex or long-form prompts, we should provide a larger text area that saves to a document. Ideally, these documents would be organizable in some way (e.g. in folders: #332) so they don't clutter up a user's document list too much.

We can seed some initial long-form questions to use here as prompts, but there's a lot of cool inferencing we can do from content pages (e.g. How did X meet Y? Where did Z find ?, etc)

The issue does not name files or tests. Start by tracing how prompts are tied to empty content-page fields and how documents are saved; done would require a larger prompt area backed by a document, with a way to organize those documents and initial long-form questions.

Written by the indexing model from the issue text.