Display group/project access requesters separately in admin

## What does this MR do?

It displays the access requesters in a separate list in group & project members pages.

It also harmonize the members counter UI to use `%span.badge` everywhere (in the admin & non-admin members views).

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

No.

## Why was this MR needed?

To not confuse access requesters with actual members.

## What are the relevant issue numbers?

Closes #18871.

## Screenshots

### Group members

| Before | After |
| --------- | ---- |
| ![group-members-before](/uploads/2f15137e073fd3a63bc2cb7b2217cb6c/group-members-before.png) | ![group-members-after](/uploads/5b643974505cfa57783fa0320d3bf8b2/group-members-after.png) |

### Project members

| Before | After |
| --------- | ---- |
| ![project-members-before](/uploads/9c48dcd3736e42de84061b1201ee0b06/project-members-before.png) | ![project-members-after](/uploads/8e04c92ef0bba3de7e2405618632b27d/project-members-after.png) |

### Admin group members

| Before | After |
| --------- | ---- |
| ![admin-group-members-before](/uploads/7fda8c2c94b697bea6655ba892ba45e7/admin-group-members-before.png) | ![admin-group-members-after](/uploads/ea25717001794f75939c679b80308c3a/admin-group-members-after.png) |

### Admin project members

| Before | After |
| --------- | ---- |
| ![admin-project-members-before](/uploads/ba9d3ec52adbda6bb3d45ad9ac5243d3/admin-project-members-before.png) | ![admin-project-members-after](/uploads/3b889a029a9756e9ed2781b45c4dd9cb/admin-project-members-after.png) |

## Does this MR meet the acceptance criteria?

- [x] No CHANGELOG since this is related to the original "request access" MR.
- [ ] 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 !4798
Signed-off-by: Rémy Coutable <remy@rymai.me>