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

Some contributor documentation

Don't chown -R a user's home directory

If people have a use case for recursive chowns that's fine.  I think it's dangerous for people to use, but that's tailored to my use case.

My attempt at reorganization

Made language stronger on this for variety.

extraneous space removed