Update docs about NB_USER set up

Update jupyter_notebook_config.py to fix bug introduced in notebook 5.7.0

Latest release of notebook (5.7.0) introduces bug when `c.NotebookApp.ip` is set to '*'. See https://github.com/jupyter/notebook/issues/3946 for details.

Update Juilia version to 1.0.0

Force LF line endings

Add a posibliity for a pre-start hook

- This is initial prototype of a more flexible hooks system.
- Add /usr/local/bin/before-notebook.d hooks which is run right before
  the notebook runs.

- The start.sh script does different operations on users, and
  sometimes one may need to do operations *after* that is done (as
  opposed to /usr/local/bin/start-notebook.d/ which is run before).
  This provides that possibility.  (One use case is hooks which must
  be run after the users are set up.)
- To be technically perfect, one might want to use something similar
  to /usr/local/bin/pre-start-notebook.d/.  But let's start with the
  simplest thing possible for now - the expected use of this command
  is only if people use NB_USER, etc in a way that requires small
  follow up commands to be run.
- If notebook is running as root, this allows users to run arbitrary
  commands pre-sudo.  Consider security model.

Update jupyterlab & extensions

Update JupyterHub to 0.9

Update group handling: set primary gid, leave suplemental with group users

[ci skip] Switch docs to the alabaster theme

- If NB_GID is set, it is likely that files created should have NB_GID
  as their gid (if it doesn't matter, then this change is irrelevant.
  If it does matter, the liklihood that 100 matches with what the user
  wants is low).
- When NB_GID is set, create a new group with that gid.  Then, set
  this as the primary gid of the user.  Set the user's supplemental
  gids to NB_GID,100.

Bump up jupyterlab & extensions version

point README badges to dockerhub

Right now URLs simply point to the badge image source

Install tini from conda-forge

Change XDG_CACHE_HOME when using NB_USER

Switches to getting `tini` from `conda-forge`.

Update version of seaborn to 0.9.X

Adds config file initialization and updates Apache Mirror due to timeout issue

Adds the initial creation of the configuration file
jupyter_notebook_config.json to the image. This file
was missing as reported in issue #639 and caused an
error when trying to set a new password via the Web
interface.

Bumps the version of Jupyter Notebook to 5.6

Updates Apache Mirror URL because the current causes build timeouts

The mirror used now (mirrors.ukfast.co.uk) is fast enough and does not
point to an ad site and is already used in a different Dockerfile within
this repository.

[ci skip] Remove dead files w/ old versions

Fixed README, Jupyter Lab launch sample command

Fix #492: NB_GID adds a supplemental group, leaves primary untouched