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/plugins/layout/index.js

@@ -106,18 +106,18 @@ type SearchResultTree = {|
   axElement: Element,
 |};
 
-const LoadingSpinner = LoadingIndicator.extends({
+const LoadingSpinner = styled(LoadingIndicator)({
   marginRight: 4,
   marginLeft: 3,
   marginTop: -1,
 });
 
-const Center = FlexRow.extends({
+const Center = styled(FlexRow)({
   alignItems: 'center',
   justifyContent: 'center',
 });
 
-const SearchIconContainer = styled.view({
+const SearchIconContainer = styled('div')({
   marginRight: 9,
   marginTop: -3,
   marginLeft: 4,
@@ -131,7 +131,7 @@ class LayoutSearchInput extends Component<
     value: string,
   },
 > {
-  static TextInput = styled.textInput({
+  static TextInput = styled('input')({
     width: '100%',
     marginLeft: 6,
   });