* Add RStudio default settings for handling history (#79)

Added to Dockerfiles:

- mkdir /home/rstudio/.rstudio/monitored/user-settings

- create file /home/rstudio/.rstudio/monitored/user-settings/user-settings

	- alwaysSaveHistory = FALSE
	- loadRDate         = FALSE
	- saveAction        = FALSE

- chown rstudio /home/rstudio/.rstudio (Must be done else RStudio will not
  load; generates error message).

Originally, this was added to all Dockerfiles in subdir rstudio. However, on
tidyverse builds these settings were overwritten somehow (user-settings file
existed but contents were slim and not equal).

Added similar section to all tidyverse Dockerfiles.

Sample test builds of rstudio and tidyverse images verify settings work as
expected.

* Add RStudio user-settings file

* Modify commit @00bd6da, COPY user-settings to img

Undo changes to RStudio Dockerfiles in commit 00bd6da.

COPY user-settings (commit de308a4) to image on build.

Change ownership of ~/.rstudio directory to rstudio to prevent "Error occured
during the transmission" errors that will prevent RStudio from loading.

See: #79, #80

* Undo tidyverse changes in commit 00bd6da

* Add PAM helper script for Singularity

* Rename auth_singularity.sh to pam-helper.sh, and add to devel image

RStudio Server version in rocker/rstudio is specified as latest version available at default MRAN date of rocker/r-ver.

* Change version of libssl since the older one got archived

* Added update to all 3.4.x versions

pandoc-templates archive 1.18 no longer exists. just get latest version of
pandoc templates instead

(Debian 9.0, release 2017-06-17) for 3.4.0, latest, and devel

* test for building newer versions of r-ver on debian:stretch
* setup newer rstudio images for stretch
* lock older versions to jessie, fix PKI on stretch versions (requires dev version from r-forge)
* use dev version of `littler` `install2.r` script to avoid erroneous error on warning
* Mention stretch vs jessie use in README

Appears that R reads only Rprofile.site but not an Rprofile file in
R_HOME/etc.  However, R installation creates a Renviron in R_HOME/etc,
not an Renviron.site.  It appears also that RStudio copies these into
/etc/R, and renames both with the .site suffix.  If they do not exist,
or /etc/R does not exist, it does not end up reading Renviron at all.
Pretty confusing to me...