## Summary

This PR cleans up the `kbn-grid-layout` code in two ways:
1. Rather than memoizing components in their parents, I swapped to using
`React.memo` for all components, which accomplishes the same behaviour
in a slightly cleaner way.
2. I moved all Emotion style definitions **outside** of the React
components so that we no longer have to re-parse the CSS string on every
render (see [this
comment](elastic/eui#6828 (comment))).

### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

(cherry picked from commit 219f31a)

# Conflicts:
#	src/platform/packages/private/kbn-grid-layout/grid/grid_panel/resize_handle.tsx