Merge branch 'update-button-style' into 'master' ldap_user_filter_update

Create secondary colors for buttons with SCSS functions [Proper]

## What does this MR do?
* Removes the hardcoded values for button colors and generates them using the `darken` function
* Adds a border color for outline buttons on hover, focus and active states
* Adds darker colors for outline buttons in their active state

## Are there points in the code the reviewer needs to double check?
Naming for new factor variables might not be good enough.

## Why was this MR needed?
Increases visibility of alternate states as specified in #24145

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #24145

See merge request !7797

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