Upgrade to emotion v10

Summary: React 16 is not compatible with react-emotion 9 (it prints warnings, see also https://github.com/emotion-js/emotion/issues/644). So we should upgrade to 10.

Reviewed By: mweststrate

Differential Revision: D18905889

fbshipit-source-id: c00d2dbbadb1c08544632cb9bfcd63f2b1818a25

src/ui/components/VerticalRule.tsx

@@ -7,9 +7,9 @@
  * @format
  */
 
-import styled from 'react-emotion';
+import styled from '@emotion/styled';
 
-const VerticalRule = styled('div')({
+const VerticalRule = styled.div({
   backgroundColor: '#c9ced4',
   width: 3,
   margin: '0',