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)

r-notebook/Dockerfile

@@ -32,4 +32,6 @@ RUN conda install --quiet --yes \
     'r-caret=6.0*' \
     'r-rcurl=1.95*' \
     'r-crayon=1.3*' \
-    'r-randomforest=4.6*' && conda clean -tipsy
+    'r-randomforest=4.6*' && \
+    conda clean -tipsy && \
+    fix-permissions $CONDA_DIR