Remove root claim from back-end

[keyserj]

Is your feature request related to a problem? Please describe. There's a decent amount of logic that's a bit jank and exists solely to support root claim nodes (e.g. to create one if it doesn't exist when viewing the claim tree, or when adding a claim, or preventing viewing the claim tree if the root node doesn't exist). We shouldn't need to do this.

Describe the solution you'd like

• Have supports/critiques point (in data) directly at the parent node/edge so that we don't need to create an extra node to make this connection

Questions:

• Can we get rid of arguedDiagramPartId as well?
• Should we just replace the root node UI with something else anyway, rather than just replacing in data? (e.g. showing the parent node or edge in the diagram directly)

Describe alternatives you've considered

Additional context

• Ensure that deleting a parent criteria will result in deletion of the deleted edges' claim trees
• Some filters rely on iterating over all of a node's children, make sure these don't break if claims become direct children

Technical ideas

[keyserj]

Currently think we should replace this with "supports importance of" and "critiques importance of" relations directly to a structure node/edge. Would need to make edge->edge relations work, with some UX to see edge supports/critiques (maybe just don't show them in the diagram, and don't have a justification tree for these...?)