Avoid recoloring any old icon inside Link

[ry5n]

WHY are these changes introduced?

Fixes #1724

WHAT is this pull request doing?

The Link component is primarily intended for text links, but it can and is used for links around images and other content. Currently, all icons inside Link are set to the current text color, which doesn’t make sense except for text links

This problem was introduced in #1247 which adds an icon automatically for external links.

The fix is to scope the icon color change to the external link case only.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

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

import * as React from 'react';
import {ImageMajorMonotone} from '@shopify/polaris-icons';
import {Page, TextContainer, Link, Icon} from '../src';

interface State {}

export default class Playground extends React.Component<{}, State> {
  render() {
    return (
      <Page title="Playground">
        <TextContainer>
          <p>
            <Link url="#">
              <Icon source={ImageMajorMonotone} />
            </Link>
          </p>
          <p>
            <Link url="https://help.shopify.com/manual" external>
              Shopify Help Center
            </Link>
          </p>
        </TextContainer>
      </Page>
    );
  }
}

🎩 checklist

• [N/A] Tested on mobile
• Tested on multiple browsers
• [N/A] Tested for accessibility
• [N/A] Updated the component's README.md with documentation changes
• [N/A] Tophatted documentation changes in the style guide

[BPScott]

LGTM 👍

[carysmills]

Code looks good to me!
Thanks for fixing this!