Upgrade user image

• Bump to Ubuntu 20.04. This brings in lots of newer packages, but particularly Python 3.8 (https://phabricator.wikimedia.org/T265957). Many new packages are 3.7+ only.
• Use upstream R deb packages (https://cran.r-project.org/bin/linux/ubuntu/), than those from Ubuntu. This is maintained by the same folks who do R in debian. This brings us R 4.x (https://phabricator.wikimedia.org/T268923), but more importantly enables binary package installation.
• Use packagemanager.rstudio.com to install R packages. This is a huge benefit - it can install any version of packges, and it has binary package builds for most of them! These binary packages are built against focal + the upstream R debs. See announcement info here: https://blog.rstudio.com/2020/07/01/announcing-public-package-manager/. This reduces the user image build time a *lot*
• Put R packages in /srv/r, to match /srv/paws for our venv. Users can more easily install R packages temporarily now, and we don't mess with things under /usr. It also simplifies our Dockerfile, reducing number of times we have to switch to root.
• Redirect most stdout from apt-get to /dev/null. We still get stderr, so no real functionality lost.
• Remove some explicit nbextension enable / install commands. These are no longer necessary.
• Specify a loose pin for notebook & jupyterlab versions.