This reverts commit 8751491b.

- Tab between register and sign in forms
- Add individual input validation error messages
- Validate username
- Update many styles for all login-box forms

Enforce TrailingSemicolon and EmptyLineBetweenBlocks in scss-lint

## What does this MR do?
Enforces two new scss-lint rules
1.  TrailingSemicolon
2.  EmptyLineBetweenBlocks

## Are there points in the code the reviewer needs to double check?
Make sure I didn't break anything 😁 

## Why was this MR needed?
Improve our SCSS, Lessen the burden of catching these during the review process

## Screenshots (if relevant)
NOne

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #23319

See merge request !6903