convert to emotion

Summary:
My benchmarks have shown react-emotion to be faster than the current implementation of `styled`. For this reason, I am converting all styling to [emotion](https://emotion.sh).

Benchmark results:
{F136839093}

The syntax is very similar between the two libraries. The main difference is that emotion only allows a single function for the whole style attribute, whereas the old implementation had functions for every style-attirbute.

Before:
```
{
  color: props => props.color,
  fontSize: props => props.size,
}
```

After:
```
props => ({
  color: props.color,
  fontSize: props.size,
})
```

Reviewed By: jknoxville

Differential Revision: D9479893

fbshipit-source-id: 2c39e4618f7e52ceacb67bbec8ae26114025723f

src/ui/components/VirtualList.js

@@ -8,33 +8,24 @@
 import FlexColumn from './FlexColumn.js';
 import {Component} from 'react';
 import View from './View.js';
+import styled from '../styled/index.js';
 
-const Inner = FlexColumn.extends(
-  {
-    alignItems: 'flex-start',
-    height: props => props.height,
-    minHeight: '100%',
-    minWidth: '100%',
-    overflow: 'visible',
-    width: '100%',
-  },
-  {
-    ignoreAttributes: ['height'],
-  },
-);
+const Inner = styled(FlexColumn)(({height}) => ({
+  alignItems: 'flex-start',
+  height,
+  minHeight: '100%',
+  minWidth: '100%',
+  overflow: 'visible',
+  width: '100%',
+}));
 
-const Content = FlexColumn.extends(
-  {
-    alignItems: 'flex-start',
-    height: '100%',
-    marginTop: props => props.top,
-    minWidth: '100%',
-    overflow: 'visible',
-  },
-  {
-    ignoreAttributes: ['top'],
-  },
-);
+const Content = styled(FlexColumn)(({top}) => ({
+  alignItems: 'flex-start',
+  height: '100%',
+  marginTop: top,
+  minWidth: '100%',
+  overflow: 'visible',
+}));
 
 type VirtualListProps = {|
   data: Array<any>,