Allow adm group to view Jellyfin resources

This is pretty arbitrary and personal, but I detest services that
lock out global access (which is good), but don't simultaneously
make it easy for administrators to enter the directories. The adm
group should only have actual system administrators as members,
so this lets them view the secure directories.

deployment/debian-package-x64/pkg-src/postinst

@@ -43,7 +43,8 @@ case "$1" in
       mkdir $CACHEDATA
     fi
     # Ensure permissions are correct on all config directories
-    chown -R jellyfin:jellyfin $PROGRAMDATA $CONFIGDATA $LOGDATA $CACHEDATA
+    chown -R jellyfin $PROGRAMDATA $CONFIGDATA $LOGDATA $CACHEDATA
+    chgrp adm $PROGRAMDATA $CONFIGDATA $LOGDATA $CACHEDATA
     chmod 0750 $PROGRAMDATA $CONFIGDATA $LOGDATA $CACHEDATA
 
     chmod +x /usr/lib/jellyfin/restart.sh > /dev/null 2>&1 || true