Introduce a global tutor-password

Description / Overview

Tutors can currently register with any name. Students with enough evil intentions could try to register with their tutor's name to eventually get access if one is not careful. We'd need a global tutor password that is kept confidential for all tutors and is set per-instance.

Feature checklist

• Set tutor-password during import phase
• Validate tutor-password on registration
• Add field to send tutor-password on registration form

(Make sure these are completed before closing the issue, with a link to the relevant commit.)