WIP added static paths to PathHelper and using them in wp templates.

https://www.openproject.org/work_packages/13603
https://www.openproject.org/work_packages/13697

@myabc I played a bit with your idea of using URI templating in PathHelper but am not sure if it would make the code that much cleaner. Maybe if you had some specific ideas you could let me know as to me it seems like we'd end up in a similar situation of building up URI directory strings and then feeding them into URI constructors and not really using and of the other URI functionality after that.