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

Require at least blas 1.1; otherwise other packages dependencies may
attempt to downgrade it.

Add Mesos GPG key since accessing the keyserver seems to be flaky:
https://github.com/docker-library/official-images/issues/4252#issuecomment-381783035

Upgrade Ubuntu to latest LTS (Bionic 18.04), including:
- Change the IMAGE used for base-notebook;
- Keep Mesos install from Mesosphere's Xenial repository, since they
  don't yet have a Bionic repository;
- gnupg needs to be installed for apt-key to work;
- --force-yes option to apt-get has been removed;
- Package libav-tools is replaced by ffmpeg;
- Update Let's Encrypt example script to use 18.04.

Upgrade to conda 4.5.4

[ci skip] Fill out github community files

Add nano and vim to Dockerfiles

Migrate from READMEs to RTD site