Support changing the locale

Currently there is no way to change the locale with the dev-harness. Seems like everything is in place for it, but there's no UI yet.

```tsx
/* AppHeader/index.tsx */

import React, { ChangeEvent } from 'react';
import { Box, MenuItem, TextField } from '@alteryx/ui';

/* ... */

const AppHeader = (props: IAppHeaderProps): JSX.Element => {
/* ... */
const handleChangeLocale = (event: ChangeEvent): void => {
handleSetLocale(event.target.value);
};

return (
/* ... */


{localeOptions.map(option => (

{option.primary}

))}

)}
label={Locale}
labelPlacement="start"
/>

/* ... */
);
};
```