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