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] Begin of the insertionDependency lines betweenEnd of the insertion work Begin of the insertionpackages (follows-precedes relationships)End of the insertion Begin of the deletion package sortingEnd of the deletion
* Begin of the insertion[x]End of the insertion Begin of the deletion [ ]End of the deletion work package Begin of the insertionsortingEnd of the insertion Begin of the deletion hierarchiesEnd of the deletion
* [ ] work package Begin of the insertionhierarchiesEnd of the insertion Begin of the deletion groupingsEnd of the deletion
* [ ] Begin of the deletion Dependency lines betweenEnd of the deletion work Begin of the insertionpackage groupingsEnd of the insertion Begin of the deletion packages (follows-precedes relationships)End of the deletion

Begin of the insertionOut of scope:End of the insertion Begin of the deletion Rejected:End of the deletion
\* Export work package table (on the left) as well as Gantt chart (on the right)