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

Fix admin appearance settings preview

Render appearance settings preview in devise layout instead of implementing it 2 times. 

See merge request !4792

Also, reuse partials from the non-admin views.

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

* upstream/master: (353 commits)
  Put some admin settings in dropdown
  Add styleguide on configuration settings documentation
  Remove Duplicated keys add UNIQUE index to fingerprint
  Avoid autoload issue such as 'Mail::Parsers::AddressStruct'
  Move appearance settings as sub tab to application settings
  use rails root join
  fixed a couple of errors spotted in production
  Fix RangeError exceptions when referring to issues or merge requests outside of max database values
  Fix bug in `WikiLinkFilter`.
  Grammar and typographic changes to artifacts documentation
  Tweak grammar
  Small frontend code fixes and restore 8a2d88f commit
  Warn about admin privilege to disable GitHub Webhooks
  Listing GH Webhooks doesn't stop import process for non GH admin users
  fixup! updated docs for api endpoint award emoji
  Update CHANGELOG
  Ensure Todos counters doesn't count Todos for projects pending delete
  Add endpoints for award emoji on notes
  Sort API endpoints and implement feedback
  Add endpoints for Award Emoji
  ...

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>