Render many more loading placeholders than required in Media List

Previously enough loading placeholders were rendered to cover the end of the row. However, users were reporting that the external media library loading was slow. After discussion design input suggested loading lots of placeholders than necessary would improve the perception of speed. This has been implemented.

See
* #22681
* #25581

client/my-sites/media-library/list.jsx

@@ -196,8 +196,12 @@ export class MediaLibraryList extends React.Component {
 
 	renderLoadingPlaceholders = () => {
 		const itemsPerRow = this.getItemsPerRow();
+		// Factor is added in response to users reporting that photos appeared
+		// to take a long time to load. More indicators are loaded than are necessary
+		// as this improves the perception of speed for the user
+		const factor = 4;
 		const itemsVisible = ( this.props.media || [] ).length;
-		const placeholders = itemsPerRow - ( itemsVisible % itemsPerRow );
+		const placeholders = itemsPerRow * factor - ( itemsVisible % itemsPerRow );
 
 		// We render enough placeholders to occupy the remainder of the row
 		return Array.apply( null, new Array( placeholders ) ).map( function( value, i ) {