[FEEDS] - Added Flickr and Youtube support

Supersede #5646

Remaining issues:
* [x] The Flickr and YouTube don't work at the same time, it's always on or the other.
* [ ] When selecting the YouTube tab then navigating to another tab, the second tab doesn't show up (requires double clicking to work)
* [ ] Only the first video player loads properly in the lightbox
Avoid multiple return statements
```js
if ( toProcess === 1 ) {
	parseEntries( entries, $content.data( "feedLimit" ), $content, feedtype );
	toProcess = 0;
} else {
	toProcess -= 1 ;
	$content.data({
		"toProcess": toProcess,
		"entries": entries
	});
}

return toProcess;
```