Improve the request / withdraw access button

## What does this MR do?

It implements the design proposed in #18310.

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

No.

## Why was this MR needed?

To close #18310.

## What are the relevant issue numbers?

Closes #18310.

## Screenshots

### Group access request

#### Request button

| Medium | Large |
| ----------- | ------- |
| ![request_access_button](/uploads/a1de370dcbb8ac9a63d2df5c68591db7/request_access_button.png) | ![request_access_button-large](/uploads/0a1c70380268e620a6ca4d3e1661d58c/request_access_button-large.png) |

#### Withdraw request button

| Medium | Large |
| ----------- | ------- |
| ![withdraw_access_request_button](/uploads/c9df39d04b61566ec143d5e9cc43ada2/withdraw_access_request_button.png) | ![withdraw_access_request_button-large](/uploads/10fdaa94d72956e06bdb995e65b51472/withdraw_access_request_button-large.png) |

### Project access request

#### Request button

| Medium | Large |
| ----------- | ------- |
| ![request_access_button](/uploads/8e71395041a5cea996a35df2083bb723/request_access_button.png) | ![project-request_access_button-large](/uploads/adb2dec0eccec8e5171dc0e26e6b03a6/project-request_access_button-large.png) |

#### Withdraw request button

| Medium | Large |
| ----------- | ------- |
| ![withdraw_access_request_button](/uploads/12be06f0a2bf9426a5e043f52c4d1dab/withdraw_access_request_button.png) | ![project-withdraw_access_request_button-large](/uploads/93fda7767ee5f02186c4c954ea346254/project-withdraw_access_request_button-large.png) |

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] 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 !4860