Refactor: Leaderboard Grid

[briangregoryholmes]

This PR removes the use of the VirtualizedGrid component and instead relies on the IntersectionObserver API to lazy load Leaderboard queries.

This was done to reduce code complexity and align the implementation with the lazy loading approach that will be taken with the time series charts.

Additionally, the loading state of the leaderboards has been changed the following:

This is not a requirement of the PR and is open to feedback, but it does solve two related issues:

1. The scrollbar no longer erratically resizes as data "beneath the fold" is fetched
2. For most leaderboards, the vertical height of the loading state now more closely matches the loaded state

This is somewhat of a stopgap UI improvement and loading states for the Leaderboards will be further refined once they're converted to HTML tables.

[ericpgreen2]

I like it. The taller loading state also improves loading performance, especially for big Druid-backed dashboards. Currently when a dashboard is opened, we unnecessarily fire toplist queries for bottom-of-the-screen leaderboards that are ultimately pushed off-screen & unmounted when top-of-the-screen leaderboards' data arrives.