[EuiDataGrid] UX Cleanups

[chandlerprall]

Originally from #4170 (review)

1. The density adjustments no longer seem to be changing the overall height of the cells. Resolved in #5372
There seems to be some calculations going that always come out to make the cells 34px high.

2. In full screen mode, the initial calculation doesn't take the scrollbar into account Resolved

3. Visually I think we need more borders especially around the scrollbars Resolved in #5563

4. It would be great if while scrolling with virtualization which shows a white background while the cells load, we could have either add in some light gray like $euiPageBackgroundColor, or even some sort of pattern indication "loading". Resolved by #5562

5. It looks like we also need a smarter pagination display on skinny screens/containers. Resolved by #5561

6. In full screen, the bottom row is still getting cutoff by the fixed pagination and it's hiding the horizontal scrollbar. Different densities are cutoff by different amounts. Resolved by #5557

[cee-chen]

Closing this out, all individual items should have been resolved!