Allow automatic layout of the workflow graph

By using an automatic graph layout algorithm the workflow graph can be nicely arranged at the press of a button.
The user should still be able to rearrange the graph afterwards.

We will rely on dagre (https://github.com/cpettitt/dagre) to lay out the graph on the client-side.
We will also switch to dynamic or perimeter anchors in JSPlumb (http://jsplumb.org/doc/anchors.html#dynamic) to reduce edge crossing.