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 |
| --------- | ---- |
|  |  |
### Project members
| Before | After |
| --------- | ---- |
|  |  |
### Admin group members
| Before | After |
| --------- | ---- |
|  |  |
### Admin project members
| Before | After |
| --------- | ---- |
|  |  |
## 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
