Don't store the admin password in cleartext

Originally created by @segfault on #17135 (Redmine)

The Greeter currently stores the user-chosen admin password unhashed in /var/lib/gdm3/tails.password. In /etc/gdm3/PostLogin/Default, the password is then set via chpasswd and /var/lib/gdm3/tails.password is removed.

IMO, passwords should never be stored in cleartext. Instead, we should store them hashed and use chpasswd -e to set them.

This will also make it easier to persist the password, as part of persisting the Greeter options, which I plan to work on.

Feature Branch: bugfix/17135-store-admin-pw-hashed