Added video and audio support

Added support for video files (only webm, ogv, mp4) and audio files (only ogg, mp3, wav) to be embedded with html5 video and audio elements. This uses the media syntax that already exist for creating just a link for them.
To ensure cross-browser compatibility all files with the same filename are automatically chosen as alternative sources when only one of them is mentioned in the media syntax. Additionally, the video supports a "poster" image which will be shown before the video starts. That poster also needs to have the same filename but with an jpg or png extension.

What still needs to be done:
* Video file icons could be better and differentiate between the different formats. (nice-to-have)
* ~~Audio and video elements are currently within a paragraph, which is invalid html. Fixing that is a bit more complicated and should be done by a more "backendy" person. (must-have)~~ *Turned out not to be true.*