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/leak_canary/index.js

@@ -16,6 +16,7 @@ import {
   Checkbox,
   SonarPlugin,
   Button,
+  styled,
 } from 'sonar';
 import type {ElementID, Element} from 'sonar';
 import {processLeaks} from './processLeakString';
@@ -42,12 +43,12 @@ export type Leak = {
   retainedSize: string,
 };
 
-const Window = FlexRow.extends({
+const Window = styled(FlexRow)({
   height: '100%',
   flex: 1,
 });
 
-const ToolbarItem = FlexRow.extends({
+const ToolbarItem = styled(FlexRow)({
   alignItems: 'center',
   marginLeft: '8px',
 });