packagemanager.rstudio.com is a CRAN mirror provided
by rstudio, with *binary packages* prebuilt for many Linux
Distributions! https://www.rstudio.com/blog/announcing-public-package-manager/
has more excellent detail. It cuts down install times for R packages
by almost 90% in some cases!

Like MRAN (which we use now), they also provide a daily snapshot
of CRAN at that date
(https://docs.rstudio.com/rspm/news/#rstudio-package-manager-2021090).
The URL for CRAN for a particular date can be fetched via an API
call. We call that API, and if there is no snapshot for that date
(anything before Oct 2017), we fall back on to MRAN. Adds a test
to test this fallback.

One possible issue about changing existing binder repos to use binary
builds rather than source builds is that the binary builds sometimes
require you have an apt package installed, and will fail if it is
not. We had to install the zmq library apt package for example -
source installs compile zmq from source, which is where the speedup
comes from. But unlike python wheels or conda packages, these binary
builds are not self-contained - they are linked to apt packages from
the specific distros. So some repos that worked before might fail now.
We can choose a more recent cut-off date to prevent this from happening.

We were doing this from an old MRAN snapshot. I moved the pin
a little ahead, so IRKernel can also be installed from CRAN
instead of from GitHub. R <= 4.0 gets the old version, and anything
newer gets a more recent version of devtools. This gives us
fast installs for IRkernel with binary packages.

Also add a R 4.0 and R 4.1 test

- Install a different version of RStudio for R < 4.1,
  as latest RStudio doesn't seem to support those. And
  newer RStudio isn't supported on these older R versions.
- Cleanup how Shiny is installed - install it with the same
  apt invocation as rstudio (saves time), and install shiny-proxy
  from PyPI instead or GitHub. The release on PyPI is the same
  as our previous GitHub pin.
- Remove outdated comment about different behavior for R 3.6 - I
  think now we get all our R versions from the same apt repo. Plus,
  the conditional was adding more scripts than just adding extra apt
  package repos

- MRAN doesn't seem to have R 4.1 specific snapshots, so let's
  default to RSPM for anything 4.1+.
- Otherwise, snapshot dates in 2022 will result in using rspm

Otherwise latest version was being installed, giving us
R 4.1 even when we ask for 4.0

And add another R test for R4.0 + rspm

Based on output of apt-cache madison r-base-core

Was accidentally included along with the 3.6.3-1biocnic
upgrade for 3.6