Point badges to dockerhub, add alt-text to microbadger

* status, pulls badges now point to https://hub.docker.com/r/jupyter/tensorflow-notebook/
* microbadger badges now have 'image metadata' alt text

* status, pulls badges now point to https://hub.docker.com/r/jupyter/scipy-notebook/
* microbadger badges now have 'image metadata' alt text

* status, pulls badges now point to https://hub.docker.com/r/jupyter/r-notebook/
* microbadger badges now have 'image metadata' alt text

* status, pulls badges now point to https://hub.docker.com/r/jupyter/pyspark-notebook/
* microbadger badges now have 'image metadata' alt text

* status, pulls badges now point to https://hub.docker.com/r/jupyter/datascience-notebook/
* microbadger badges now have 'image metadata' alt text

* status, pulls badges now point to https://hub.docker.com/r/jupyter/all-spark-notebook/
* microbadger badges now have 'image metadata' alt text

Updates:

* Currently clicking on the pulls & stars badges directs users to the actual button svg image source. The link should point to https://hub.docker.com/r/jupyter/minimal-notebook/.
* Add alt-text "image metadata" to microbadger badge

Fix builds by updating to compatible versions

The RC1 contains the fix to support latest release of JupyterLab.

Fixes #710

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