mobian-tweaks: allow numeric passwords (minlen=4)
/etc/pam.d/common-password demands a non-numeric and complex password which is suboptimal as it is used for the lock screen (which for now can only take digits). So override /etc/pam.d/common-password to alter the requirements. 4-digit numbers are ok now.
ref: https://wiki.ubuntu.com/PAMConfigFrameworkSpec
Credits to Mohammed_Sadiq @ purism who implemented this for the Librem5.
Fixes Issue 9
Debian installation scripts have been adopted to remove everything cleanly if we remove the tweaks package and go back to the original.