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/sandbox/index.js

@@ -20,13 +20,13 @@ type SandboxState = {|
   showFeedback: boolean,
 |};
 
-const BigButton = Button.extends({
+const BigButton = styled(Button)({
   flexGrow: 1,
   fontSize: 24,
   padding: 20,
 });
 
-const ButtonContainer = FlexColumn.extends({
+const ButtonContainer = styled(FlexColumn)({
   alignItems: 'center',
   padding: 20,
 });
@@ -42,7 +42,7 @@ export default class SandboxView extends SonarPlugin<SandboxState> {
   static id = 'Sandbox';
   static icon = 'translate';
 
-  static TextInput = styled.textInput({
+  static TextInput = styled('input')({
     border: `1px solid ${colors.light10}`,
     fontSize: '1em',
     padding: '0 5px',
@@ -52,13 +52,13 @@ export default class SandboxView extends SonarPlugin<SandboxState> {
     flexGrow: 1,
   });
 
-  static FeedbackMessage = styled.text({
+  static FeedbackMessage = styled('span')({
     fontSize: '1.2em',
     paddingTop: '10px',
     color: 'green',
   });
 
-  static TextInputLayout = FlexColumn.extends({
+  static TextInputLayout = styled(FlexColumn)({
     float: 'left',
     justifyContent: 'center',
     flexGrow: 1,