Use all_cleaned_data instead of seperate cleaned_data per step

See merge request !40

Currently translated at 100.0% (285 of 285 strings)

Translation: Hack'n'Fun/AlekSIS-App-Paweljong
Translate-URL: https://translate.edugit.org/projects/hacknfun/aleksis-app-paweljong/de/

Resolve "Display individual price at registration with voucher"

Closes #45

See merge request !37

Connect email to person on account registration. Closes #44

Closes #44

See merge request !39

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 !34

Resolve "Mark as payed on registration overview page"

Closes #31

See merge request !33

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

Closes #40

See merge request !32

Resolve "Checkpoint system"

Closes #33

See merge request !30