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

This reverts commit bf2a86b73cce332ff8f4392ffc8df501193f32ec.

This reverts commit 68155ee73b549a4f79744bb325542c29d45c71ea, reversing
changes made to 7ebd011ed1de7aee706f07a53c63c90f1c8aa5d4.

As requested by the issue #14508 this adds an option in the application
settings to set newly registered users by default as external. The
default setting is set to false to stay backward compatible.

Exclude requesters from Project#members, Group#members and User#members

## What does this MR do?

It excludes requesters from the `Project#members`, `Group#members` and `User#members` associations, and adds new `Project#requesters` and `Group#requesters` associations.

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

Without this, if you call `project.members`, requesters are included in the results! This is at best misleading, and at worst can lead to security issues. By excluding requesters from the `#members` associations, we avoid introducing security inadvertently since you have to call the `#requesters` association explicitly to get requesters.

## What are the relevant issue numbers?

This is something I realized while fixing the security issue #19102.

## Does this MR meet the acceptance criteria?

- [x] I don't think this needs a CHANGELOG since this is an internal change
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4946

And create new Project#requesters, Group#requesters scopes.

Signed-off-by: Rémy Coutable <remy@rymai.me>