Improve video player width

[timonegk]

Currently, the video player has always the same width of 750px. This should be increased on larger screens.

[fredericobormann]

As mentioned in #73 CSS pixels are device independent, so they are actually a good unit to refer to the display width. Although it would be nicer to use a maximum height relative to the viewport height instead. However, mixing specification for width and height seems to cause display problems. So all in all, I guess specifying pixels is on of the best solutions at the moment.

[lilioid]

I would also like the possibility to explicitly have a larger view are like YouTubes cinema mode