Add link style of external links

[samuelsidler]

In the handbooks, some links go to other handbook pages, some go to other WordPress.org pages, and some go to external pages. Talking with Drew, we should distinguish these separate types links somehow. Design-wise, I'm not sure what they should look like yet.

[ocean90]

Slack:

a[href^="https://wordpress.slack.com/"] {
    padding-left: 20px;
    background: url('https://slack.global.ssl.fastly.net/20655/img/icons/favicon-32.png') no-repeat;
    background-size: 16px 16px;
}

Globe:

a[href*="//"]:not([href*="wordpress.org"]) {
	background: url(data:image/svg+xml;charset=utf-8;base64,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) no-repeat;
	background-size: 14px 14px;
	padding-left: 15px;
}

(SVGs: ​https://github.com/WordPress/dashicons/tree/master/svg, data URI generator: ​http://dopiaza.org/tools/datauri/index.php)

Styles should be applied to .handbook .postcontent.

[samuelsidler]

In 2423:

Make P2 Theme: Add icon to all Slack links across the P2 and icon to all external links in handbooks.

Props ocean90.
Fixes #445.