Resolve "Create a tab shortcut to jump directly to content"

## What does this MR do?
Adds accessibility shortcut to page, i.e. if user hits `tab` key right after the page is loaded and user has not interacted to any part of the page, the shortcut appears at top left corner of the page. Then, pressing `enter` (`spacebar` support is WIP) scrolls page to focus on content.

## Are there points in the code the reviewer needs to double check?
See if `id` attribute added to `.content` element (present in [_page.html.haml](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/layouts/_page.html.haml)) needs to be reviewed if it is correct approach or we already have an ID that shortcut can refer to.

## Why was this MR needed?
This feature is particularly useful for screen readers where user doesn't have to go through entire nav to reach content of page.

## Screenshots (if relevant)
![Screenshot_from_2016-11-17_17-59-22](/uploads/0eb0149f46bd35e9ce629e7a6533be53/Screenshot_from_2016-11-17_17-59-22.png)

## 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 #24057

See merge request !7533