Configure users module packages after installation

Preseeding settings with debconf won't have any effect if the packages are already installed. Instead, provide an override database to dpkg-reconfigure.

Note that in the dpkg-reconfigure step, even if it's successful it will return an error code to indicate that it could not write to the override database file (because of the readonly flag). That's why I did not set check=True for this command.

I've tested an image that included this change, along with adding the users module's required packages as dependencies for freedombox-setup. I confirmed that users module and user logins were working as expected.