Allow all versions of all regs to be listed

This modifies the existing regulation listing endpoint to allow it to list all regulation part + version pairs we have.

To do that, it adds an extra field, `root` to regulation models, which is calculated at save-time. As we save a Regulation model for node in the tree, we rely on the `root` field to determine the root nodes. We could instead rely on the absence of a '-' in the label_string, but that's super hacky.

This allows a client to query which regs (and versions) are in the tool; previously they could only query for versions of a specified reg part.