add embed html button

[bitboxer]

We should offer a embed html button that generates the iframe code to embed the video page.

This is the code that should be generated:

<iframe width="560" height="420" src="https://signdict.org/embed/2615-nn" frameborder="0"  allowfullscreen></iframe>

The width and height should be calculated with the aspect ratio of the video. The width should always be 600px.