Resolve "hide "print vouchers" link when there is no vouchers"

added 8 commits

702779f2 - voucher has information about person that issued it
2026fc66 - debug print removed
8ee2c1f9 - unit tests are compliant with the new model
78116032 - methods modifying structure are replaced with methods that compute arrays that…
72c46735 - methods made static
84e25338 - there is place holder for current worker email address
0fa819f4 - disallow user to edit dropdown HTML element
a0818a46 - printing voucher link is visible only when there are vouchers to be printed

unmarked as a Work In Progress

merged

Merge request reports