Scrolling performance improvements for `FlexTable` and to a lesser extent `Grid`.

The primary change to `Grid` is that `tabIndex` will be set to `null` by default instead of `0`.
This improves repainting performance when a `Grid` is being scrolled.

This release removes the `FlexTable__truncatedColumnText` wrapper column and collapses its styles into `FlexTable__rowColumn`.
If you were depending on the former class you will want to update your dependencies.
I was on the fence about this in terms of compatibility, but I feel this is more of an internal implementation detail than it is public-facing API.

This release also changes the primary `FlexTable` cell from a flex container to a block.
This means that if you were right-aligning text within a column you will need to change from `align-items: flex-end` to `text-align: right`.
