cubicweb-vtimeline #2858026 extract createStoryJS in a separate method [validation pending]

A view should be able to redefine the way createStoryJS is called without having to copy the call method content.

prioritynormal
typebug
done in0.3.0
load0.100
load left0.000
closed by#b08b3b2e2e75 extract the 'createJSStory()' method in a factory (closes #2858026)