Import Layout Component in _app.tsx Instead of Pages

[otobot1]

Is your feature request related to a problem? Please describe.
Currently the Layout component is imported directly in the Page component(s).

Describe the solution you'd like
Remove Layout import from Page components and import in _app.tsx instead. Can replace the passed-in props either with logic that chooses from hard-coded values based on the current url, or by passing setState functions down to the Layout's children.

Describe alternatives you've considered
N/A. The current solution works, but causes the entire layout to be unnecessarily re-rendered on every client-side transition.

Additional context
Follow-up to #546. Mentioned in this comment.