Refactor installation

[t-kalinowski]

This PR refactors install_keras() and related CI tests.

Installation:

• consolidated logic and documentation between keras::install_keras() and tensorflow::install_tensorflow(). install_keras() is now a mostly thin wrapper around install_tensorflow() that also pulls some additional python packages.
• Added a documentation section about Apple Silicon support
• If patch level is not supplied, install_tensorflow() will pull the latest available patch release (e.g., "2.4" now pulls "2.4.2", previously it would pull "2.4.0"

CI:

• Removed the use of environment variables and optional function arguments as much as possible in the CI tests. The goal is for tests to closer match what will be the default in most situations.
• Add testing for additional versions of R: oldrel, devel, and 3.6

related: rstudio/tensorflow#476

[t-kalinowski]

Hello,

The code that installs tensorflow and keras has been refactored.
Please update to the latest versions of reticulate, tensorflow, and keras:

install.packages("remotes")
remotes::install_github(paste0("rstudio/", c("reticulate", "tensorflow", "keras")))
reticulate::install_miniconda() # skip this if you want to self-install conda or use venv
keras::install_keras()

Please let us know if after updating you are still encountering installation issues:
https://github.com/rstudio/tensorflow/issues/new

closes: #1244
closes: #1030 (RETICULATE_AUTOCONFIGURE=FALSE)
closes: #1014 (docs)
closes: #990 (self-installed conda)
closes: #987 (already fixed - awaiting response)
closes: #939 (awaiting response)
closes: #930 (RETICULATE_AUTOCONFIGURE=FALSE)
closes: #918 (outdated pip in self-installed virtualenv)
closes: #916 (stale, tf 1.13)
closes: #898 (windows-only progress bar issue)
closes: #835 (awaiting response)
closes: #1171 (stale, tf 1.4)
closes: #1158 (already fixed, CUDA)
closes: #1136 (info)
closes: #1132 (info)
closes: #1129 (info, windows)
closes: #1127 (info, windows)
closes: #1110 (info)
closes: #1106 (info)
closes: #1100 (windows)
closes: #1056 (windows, self-installed conda)
closes: #1051 (awaiting response, windows)
closes: #1038 (Pillow)
closes: #1233 (RETICULATE_AUTOCONFIGURE=FALSE)

related: #1154 rsconnect issue, tf missing on deployed shiny app