Revert signin tab order fix.

The history: 

> 1. @sytses created this: https://gitlab.com/gitlab-org/gitlab-ce/issues/23279
> 2. The fix was submitted here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6867
> 3. I [made](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6867#note_16993324) the same observation you did after it was merged
4. A long discussion about accessibility and UX followed, and we decided to revert all changes (https://gitlab.com/gitlab-org/gitlab-ce/issues/23698).

## Screenshots (if relevant)

![2016-11-17_20.55.22](/uploads/c81b0bbe0cd08747124531778af6273f/2016-11-17_20.55.22.gif)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [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 https://gitlab.com/gitlab-org/gitlab-ce/issues/23698

See merge request !7538