Improvements to hover/active/focus states in header items

## What does this MR do?

Adds some improvements to buttons, dropdowns, inputs, etc. in the header area.

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

I'm not sure.

## Why was this MR needed?

Requested in an issue.

## Screenshots (if relevant)

![caret](/uploads/574135282e4b2e81476fdd6dd6c1c05d/caret.gif)![hover](/uploads/15a3b8a2aa89baf20270be97c452e006/hover.gif)

## 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 #24202 


This is not complete yet. I'm new to this, so please let me know what all steps do I need to do (tests, changelog, etc.).

See merge request !7412