[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
`m` is probably still a bad variable name, but:
```js
m = data.media.m,
thumbnail = m.replace( "_m.", "_s." ),
image = m.replace("_m", ""),
```