Implemented the volume slider

I implemented the volume slider. It allows the user to set the volume of the soundcloud player from 0-100. Additionally you can mute the audio by clicking on the icon itself.

The client will use html5 local storage to remember the settings. This way the volume settings will be obeyed so that the user's speakers/ears are not surprised between launches.

One issue that I had to work around was that FontAwesome does not include an icon for mute (other than a speaker icon that has no sound waves coming out of it, but that is less wide so it makes the toolbar jump around). FontAwesome does have support for icon stacking, but when you do that, the icons are a slightly different size.

To solve this, I created an icon as a .png file. This works, but it may not be optimal. I stored it in /assets/system/mute.png for lack of a better place to put it.