Make QR code scanning more error tolerant

[at-dro]

Scanning printed QR codes from bad printing quality currently fails. Check if there are any options to make QR code scanning more tolerant to low quality printouts.

Proposed organizatorial workarounds until fix or if no fix is possible:

• Make sure printouts are verified by scanning the QR code once, especially if they are visibly low quality
• Provide QR codes to units or their superiors in a digital way (e.g., email or web interface).

[heinrichmartin]

Note that the workaround is covered by #43.