Refactor or deprecate parent-child relationships between XBlocks

[kdmccormick]

Refactor or deprecate parent-child relationships between XBlocks

[bradenmacdonald]

Should parent-child relationships still exist at/below the Vertical block?

Other than the "structural" XBlocks (course, sequential, vertical/unit, etc.), the only blocks I know of with has_children are:

• ConditionalBlock (built-in)
• SplitTestBlock (built-in, for A/B testing)
• LibraryContentBlock (built-in, may be renamed to Problem Bank or Item Pool Block)
• Problem Builder (which we may deprecate)
  • Problem Builder makes extensive use of child blocks. If we deprecated has_children, we'd need to figure out a path forward for courses that use Problem Builder, but that's been known for a while so needn't be a blocker here, just something to solve at the same time.

For the other three (conditional, split, LCB), I think they're going to be a blocker for now until we have some kind of Outline concept that can unify and replace them. But what I'd love to do is put in a restriction so that only those three blocks can have children, and attempting to do it for other blocks will raise an error. That way we can prevent people from introducing new dependencies on has_children.