Use prawn to export Gantt chart in background worker

* [x] Subject of work package in first column
* [x] "Zoom"
* [x] Day, month or quarter based view
* [x] Adjustable column sizes
* [x] Split chart on pages depending on PDF "paper" size
* [x] Vector based shapes in PDF
* [x] Work package type defines bar color
* [x] Work package type "milestone" is a diamond shape
* [x] Chart header repeats on every page
* [x] Dependency lines between work packages (follows-precedes relationships)
* [x] work package sorting
* [x] work package hierarchies (with index numbers as in PDF Report and PDF Table)
* Begin of the insertion[x]End of the insertion Begin of the deletion [ ]End of the deletion work package groupings

Out of scope:
\* Export work package table (on the left) as well as Gantt chart (on the right)