Remove kiosk mode application form

[jace]

Hasjob includes support for kiosk mode applications, where a job application can be submitted with no user login required. Data for the application comes from Peopleflow's NFC service.

However, Peopleflow is deprecated with its functionality rolled into Funnel, and we don't use NFC anymore. We've never taken the kiosk mode application feature into production.

Kiosk mode as a read-only interface is still useful, but currently broken as the entire header area is hidden.

To enable kiosk mode, visit https://hasjob.co/admin/kiosk?enable=1
To disable kiosk mode, visit https://hasjob.co/admin/kiosk?enable=0

Things to fix:

• Include header area in kiosk mode UI
• Remove code related to kiosk mode application submission
• Update JobApplication to make user_id non-nullable