[a11y] Make selected state work in windows high contrast mode

[sarahill]

WHY are these changes introduced?

Fixes https://github.com/Shopify/polaris-ux/issues/150

The selected date doesn't display in windows high contrast mode.

WHAT is this pull request doing?

Use the Windows high contrast media query to display background color for selected states.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import * as React from 'react';
import {Page, DatePicker} from '../src';

interface State {}

export default class Playground extends React.Component<{}, State> {
  state = {
    month: 1,
    year: 2018,
    selected: {
      start: new Date('Wed Feb 07 2018 00:00:00 GMT-0500 (EST)'),
      end: new Date('Wed Feb 07 2018 00:00:00 GMT-0500 (EST)'),
    },
  };

  render() {
    const {month, year, selected} = this.state;

    return (
      <DatePicker
        month={month}
        year={year}
        onChange={this.handleChange}
        onMonthChange={this.handleMonthChange}
        selected={selected}
        allowRange
      />
    );
  }

  handleChange = (value) => {
    this.setState({selected: value});
  };

  handleMonthChange = (month, year) => {
    this.setState({
      month,
      year,
    });
  };
}

🎩 checklist

• n/a Tested on mobile
• Tested on multiple browsers
• Tested for accessibility

[dpersing]

This looks rad!