use groups for managing write-access to files

- any files the user should be able to write should have group `user-permissions` with `g+rwX`
- remove `chown` from start.sh because it is no longer needed
- add `fix-permissions` script for setting the user-writable permissions on a path
- user-permissions group as GID 10000 (is there a reason for it to have a different value?)
- containers can set group with `--group-add user-writable` if they want to run with a different uid/gid
  (without -u root -e NB_UID -e NB_GID, which make this unnecessary)

base-notebook/start.sh

@@ -13,14 +13,6 @@ if [ $(id -u) == 0 ] ; then
     if [ "$NB_UID" != $(id -u $NB_USER) ] ; then
         echo "Set user UID to: $NB_UID"
         usermod -u $NB_UID $NB_USER
-        # Careful: $HOME might resolve to /root depending on how the
-        # container is started. Use the $NB_USER home path explicitly.
-        for d in "$CONDA_DIR" "$JULIA_PKGDIR" "/home/$NB_USER"; do
-            if [[ ! -z "$d" && -d "$d" ]]; then
-                echo "Set ownership to uid $NB_UID: $d"
-                chown -R $NB_UID "$d"
-            fi
-        done
     fi
 
     # Change GID of NB_USER to NB_GID if NB_GID is passed as a parameter