Fixes for installer (#7344)

* - move reqs file to contrib
- detect previously used python version
- safe extra requirements to INSTALLER_EXTRA

* add missing fi

* move site setting

contrib/packager.io/postinstall.sh

@@ -33,6 +33,7 @@ detect_envs
 detect_docker
 detect_initcmd
 detect_ip
+detect_python
 
 # create processes
 create_initscripts
@@ -45,6 +46,7 @@ update_or_install
 if [ "${SETUP_CONF_LOADED}" = "true" ]; then
   set_env
 fi
+set_site
 start_inventree
 
 # show info