* RStudio Server v1.2.1335 (and latest from now on) new path

* install new dependecies for RStudio Server: libclang-dev

* put new apt packges on their own lines

* one-liner if/then/else for the RStudio Server download URL

* fix if/then

* add missing semicolon

Just a tiny build efficiency tweak

* skip auth

* update docs and extend to versioned images

* 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.