optionally clear user data on uninstall

[indirectlylit]

ref: https://learningequality.slack.com/archives/CAWKSU7KR/p1587596593025200

requirements:

1. don't implement anything new that is against debian packaging policy
2. give users the option of wiping their home folder and content directory on uninstall. If they're the same, just give a single option. If they're different, ask two questions.
3. make the default to retain all data

This is not an immediate priority

[benjaoming]

Here is the postrm script of MariaDB (it asks questions!)

#!/bin/bash -e
. /usr/share/debconf/confmodule
if [ -n "$DEBIAN_SCRIPT_DEBUG" ]; then set -v -x; DEBIAN_SCRIPT_TRACE=1; fi
${DEBIAN_SCRIPT_TRACE:+ echo "#42#DEBUG# RUNNING $0 $*" 1>&2 }
#
# - Purge logs and data only if they are ours (#307473)
# - Remove the mysql user only after all his owned files are purged.
# - Cleanup the initscripts only if this was the last provider of them
#
if [ "$1" = "purge" ] && [ -f "/var/lib/mysql/debian-10.1.flag" ]; then
  # we remove the mysql user only after all his owned files are purged
  rm -f /var/log/mysql.{log,err}{,.0,.[1234567].gz}
  rm -rf /var/log/mysql
  db_input high mariadb-server-10.1/postrm_remove_databases || true
  db_go || true
  db_get mariadb-server-10.1/postrm_remove_databases || true
  if [ "$RET" = "true" ]; then
    # never remove the debian.cnf when the databases are still existing
    # else we ran into big trouble on the next install!
    rm -f /etc/mysql/debian.cnf
    # Remove all contents from /var/lib/mysql except if it's a
    # directory with file system data. See #829491 for details and
    # #608938 for potential mysql-server leftovers which erroneously
    # had been renamed.
    # Attempt removal only if the directory hasn't already been removed
    # by dpkg to avoid failing on "No such file or directory" errors.
    if [ -d /var/lib/mysql ]
    then
      find /var/lib/mysql -mindepth 1 \
           -not -path '*/lost+found/*'     -not -name 'lost+found' \
           -not -path '*/lost@002bfound/*' -not -name 'lost@002bfound' \
           -delete
      # "|| true" still needed as rmdir still exits with non-zero if
      # /var/lib/mysql is a mount point
      rmdir --ignore-fail-on-non-empty /var/lib/mysql || true
    fi
    rm -rf /var/run/mysqld # this directory is created by the init script, don't leave behind
    userdel mysql || true
  fi
  #DEBHELPER#
fi
# Modified dh_systemd_start snippet that's not added automatically due /etc/init.d/mysql
if [ -d /run/systemd/system ]; then
	systemctl --system daemon-reload >/dev/null || true
fi
exit 0