[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

Fix typo - Juptyer

Bumping up Spark and Conda to the current version

But stick with the same major and minor as the base miniconda installer

bump some packages

Updated recipe for pip and added conda example.

Updated the recipe for pip to include requirements.txt file.
Also included example for using conda to install packages.

- notebook 5.5
- matplotlib 2.2
- pandas 0.23
- scipy 1.1
- scikit-image 0.14
- statsmodels 0.9

Lock down ability to run su command.

Remove writable group file due to issues with being able to give su access when shouldn't be allowed.

Community stacks documentation

upgrade tini to latest version 0.18.0

upgrade tini to latest version 0.18.0 and also added the option to send signal to all child group processes as mentioned in docs

https://github.com/krallin/tini#process-group-killing

Document missing container options

Use `MatchSpec` to force `blas` from `conda-forge`

As it appears that `defaults` has added `blas` without a feature, the
`conda` solver appears to be prioritizing the removal of features over
channel priority. As a result, we leverage `MatchSpec` here to pin the
`blas` package to `conda-forge` and the `openblas` feature. This should
be respected by `conda` and thus avoid switching to the incompatible
`blas` package from `defaults`.

Upgrade Ubuntu to latest LTS (Bionic 18.04)

Continued work on contributor doc