create parent_levels_child_levels table

[davidsbailey]

See freshly updated Levels within Levels proposal for background. This is the first step toward implementing PLAT-95.

Testing story

Tests will be added in #34584

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[uponthesun]

Where did we land on the kind column? I couldn't tell from discussion in #34586.

[davidsbailey]

Where did we land on the kind column? I couldn't tell from discussion in #34586.

I can hold off a bit longer for that decision, but if that discussion spills into tomorrow then I'd like to proceed with this change and the decision on the kind column can come later.

[davidsbailey]

It sounds like we have consensus on using the kind column. I'll add it in a next PR.