Implement model editing

[talkasab]

We have a new tool in findingmodel where we can use the model editor in either a natural langauge mode (where the user types some basic instructions about how to update a model definition) or a more involved Mardkown-edit mode, where the user takes a Markdown rendering of a finding model definition and makes edits to it, which can then be reflected back in the underlying JSON definition.

The idea of this is to create an edited version of a finding model--a different kind of draft, one tied to an existing definition in the database.

Requirements

• These drafts still also need to be editable/updatable in themselves, exist in a public mode where others can offer comments.
• The display of the draft should highlight/summarize the difference between the draft version and the version that's already in the repository.
• Should still have the same draft → public → submitted status changes as the create drafts

Things to figure out

• How do comments get associated with it, mixed in with the comments on the base model
• What are the parts of the editing_draft class that we'll need to create/store? (Ongoing log of changes, commands, most recent edited Markdown... what else?)
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