Update Linux installer to preserve old R scripts upon install

[j-martens]

ACTION 1: UPDATE INSTALLER.

RRO doesn't overwrite the R installation on any unix platform. But it does overwrite the sym link that identifies the default R installation during the make install step.

Going forward, we should check for R scripts under /usr/bin/r during a Linux installation. If an existing R script is detected, we now want to move that file under a new directory called /usr/bin/oldR to make it easy for people to get back to an old version if they'd like.

ACTION 2

Let Josee know when implemented so she can write up an FAQ.

You can either access your prior installation by:

• Calling oldR from the command line
• Or, copying the oldR script to the R command as follows:
  1. Need steps here. STEPS STILL NEEDED
  2. Need any other steps.

Provide the steps to complete the text here:
https://github.com/RA-Internal/MRAN-site/issues/244