Merged PDF - option to align tasks on even pages

[spaanse]

During EGOI we had to print translations single-sided.
Otherwise the back of one task can contain another task.
Give an option to insert blank pages into the merged PDF such that each task starts on an even page.
This then ensures that each task is printed on a new sheet.

(suggestion - have the extra pages read "this page is intentionally left blank")

[gollux]

I encountered the some problem at this year's CEOI.

The "refresh" branch of this repository contains my work in progress, which includes significant changes in the printing workflow. I plan to implement double-sided printing there, too.

[gollux]

Fixed in the "refresh" branch.

I will close this issue when it's merged to master.