Added setting to set new users by default as external

## What does this MR do?
This implements the feature request #14508. It adds an option in the application settings to set new users by default as external.

## Are there points in the code the reviewer needs to double check?
Everything. Like I mentioned in the discussion of the issue my knowledge of Ruby basically doesn't exists. I tested it on my machine and it seems to work, but as I am very unexperienced in Ruby I highly recommend to  take a close look at the code.

## Why was this MR needed?
It was requested by @DouweM to work on the issue with the proposed changes by me.

## What are the relevant issue numbers?
This MR is for the issue #14508 that followed up after the implementation of #4009.

See merge request !4545