Resolve "Checkboxes for send_to_retracted and send_to_not_checked_in missing"

Closes #42

See merge request !36

Allow sending info mailings to retracted or only checked-in people

See merge request Teckids/hacknfun/AlekSIS-App-Paweljong!34

Resolve "Mark as payed on registration overview page"

Closes #31

See merge request Teckids/hacknfun/AlekSIS-App-Paweljong!33

Resolve "Add QR code scanning to person-group view"

Closes #40

See merge request Teckids/hacknfun/AlekSIS-App-Paweljong!32

Resolve "Checkpoint system"

Closes #33

See merge request Teckids/hacknfun/AlekSIS-App-Paweljong!30