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.p...