Improve tabbing usability for sign in page

## What does this MR do?
Rearranged the html elements so that the `sign in` button gets tabbed first before the other elements and used CSS to rearrange the visual layout.

I did not use `tabindex` because it does not play well with screen readers and is not recommended from an accessibility standpoint.

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

*  Need to make sure that there aren't any side effects to my code change. `.new_user` seemed to be automatically added through `form_for()` and I'm not sure if there will be other cases where `.new_user` is used where there is no sign in page with forget your password.

## Why was this MR needed?
Improves the tabbing order

## 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 #23279

See merge request !6867