groupshared: fix permissions of subdirs of release/ directory
groupshared: fix permissions of subdirs of release/ directory

- When the `release` and `release/{assignment_id}` directories are
  created, they are hardcoded to be mode=644.
- These are set deep within ipython_genutils, which is called via
  nbconvert.  ipython_genutils.path.ensure_dir_exists has a mode
  argument, but would require modification of both ipython_genutils
  and nbconvert in order to pass an argument all the way to there.
- Thus, we have to manually change directory permissions after the
  fact.

jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader