Add embed block placeholder.

Closes #551.

This PR adds a placeholder for the embed block.  Currently it does not
function and merely serves as a step forward on the UI.

**Testing Instruction:**

Open the inserter menu and add an embed block to the editor.  The embed
block should feature an input to submit the URL.  This functionality
does not yet work but serves as a placeholder.

**Note:**

Someone needs to do the translation build, as Windows breaks it.

blocks/library/embed/index.js

@@ -1,8 +1,12 @@
 /**
  * Internal dependencies
  */
+import './style.scss';
 import { registerBlock, query } from 'api';
 import Editable from 'components/editable';
+// TODO: Revisit when we have a common components solution
+import Dashicon from '../../../editor/components/dashicon';
+import Button from '../../../editor/components/button';
 
 const { attr, children } = query;
 
@@ -22,6 +26,21 @@ registerBlock( 'core/embed', {
 	edit( { attributes, setAttributes, focus, setFocus } ) {
 		const { url, title, caption } = attributes;
 
+		if ( ! url ) {
+			return (
+				<div className="blocks-embed is-placeholder">
+					<div className="blocks-embed__placeholder-label">
+						<Dashicon icon="cloud" />
+						{ wp.i18n.__( 'Embed URL' ) }
+					</div>
+					<input type="url" className="blocks-embed__placeholder-input" placeholder={ wp.i18n.__( 'Enter URL to embed here.' ) } />
+					<Button isLarge>
+						{ wp.i18n.__( 'Embed the link above.' ) }
+					</Button>
+				</div>
+			);
+		}
+
 		return (
 			<figure>
 				<div className="iframe-overlay">

blocks/library/embed/style.scss

@@ -0,0 +1,27 @@
+.blocks-embed.is-placeholder {
+	align-items: center;
+	display: flex;
+	flex-direction: column;
+	margin: -15px;
+	padding: 6em 0;
+	text-align: center;
+}
+
+.blocks-embed__placeholder-label {
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	font-weight: bold;
+
+	.dashicon {
+		margin-right: 1ch;
+	}
+}
+
+.blocks-embed__placeholder-input {
+	border-radius: 4px;
+	margin: 1.62em 0 1em;
+	min-width: 280px;
+	padding: 0.62em;
+	text-align: center;
+}