Require SSH key based authentication for faffy

[Firefishy]

Faffy (OpenStreetMap dev server) is the only OpenStreetMap with password based ssh authentication enabled.

To improve our security posture we should require all uses login using an SSH key.

• Evaluate which users still require a key.
• Communicate requirement.
• Set deadline

[pnorman]

Someone with root access will need to check which users have SSH keys set, because .ssh often has restrictive permissions.