password: Add password reminder

When creating the user account, offers the user a way to hint what
is the password, in case they don't remind it later.

Reworked slightly by Philip Withnall to use `g_auto*` and to stop
enforcing that a reminder is set. It should be optional.

Fixes: #56
2 jobs for password-reminder in 3 minutes and 17 seconds (queued for 1 second)