[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
Why `k` instead of `i`?