Timeline: Add group classname to foreground

The groups have an optional classname for styling, but the classname is not added to the content group, hence only the label div can be customized. Adding the following code will allow customizing of the content div aswell:

At vis.js line 6842 add:
util.addClassName(this.dom.foreground, className);

As a side note...I could not find the github repository from the visjs.org website, is this intentional?