Styles: Update external link icon

[ryelle]

The "external link" icon should be the up-right arrow [↗︎]. It is already used this way in the global header menu. This icon is also used on the site as a decorative element, in those spaces it's used inline as a real character. This does not change anything there.

When the external-link class is used on a link element or container (eg, paragraph), it will add the icon as a background, so that it is not underlined (and to prevent screen readers from reading it, see #94).

Fixes #64 — We've standardized on the single arrow, for both decorative and external links.

Screenshots

Before	After

How to test the changes in this Pull Request:

Try using this branch with the showcase site:

1. View a single site page
2. Below the title, the URL should have the new arrow icon

Or without a separate site:

1. Create a new post/page, add a paragraph
2. Add the external-link class to the paragraph under Advanced
3. Add a link in the paragraph content, not pointing to wordpress.org.
4. View on the front end
5. It should have the new arrow icon