Passing through rel=0 on YouTube embeddings

[techAPJ (Arpit Jalan)]

Added support for rel=0 via:

https://github.com/discourse/onebox/commit/734c5eef1e370e39f9624da8bb1534d114f4cdf6

Note that YouTube changed the behaviour for rel parameter since September 25, 2018:

After the change, you will not be able to disable related videos. Instead, if the rel parameter is set to 0 , related videos will come from the same channel as the video that was just played.