Resolve "missing design for wiki page"

### To Do:

- [x] Move page navigation from sub-menu to sidebar menu
- [x] Update wiki page header
- [x] Update wiki edit page actions and header
- [x] Update `Git access` page design
- [ ] ~~Add search to sidebar navigation~~

### Screen Shots:

#### `@media (min-width: $screen-lg)`

![Screen_Shot_2016-11-16_at_12.25.47_PM](/uploads/2efc59639ac53d69808e78a8ba65fcbf/Screen_Shot_2016-11-16_at_12.25.47_PM.png)  
![Screen_Shot_2016-11-16_at_12.25.36_PM](/uploads/bb7e93558335edf4216082cc1ac3f8f5/Screen_Shot_2016-11-16_at_12.25.36_PM.png)  
![Screen_Shot_2016-11-16_at_12.26.43_PM](/uploads/c2dce7cff05680b9a03438ec7d9967b4/Screen_Shot_2016-11-16_at_12.26.43_PM.png)  
![Screen_Shot_2016-11-16_at_12.26.05_PM](/uploads/7dbae25bfc93fb2f01cd231db0cb47d3/Screen_Shot_2016-11-16_at_12.26.05_PM.png) 

#### `@media (min-width: $screen-md)`

![Screen_Shot_2016-11-16_at_12.28.18_PM](/uploads/4fdcc8c88c38093ea5a4fe0b22b79acf/Screen_Shot_2016-11-16_at_12.28.18_PM.png)  
![Screen_Shot_2016-11-16_at_12.28.21_PM](/uploads/ce84a51fdc0623b2b192d8334c2c2f6d/Screen_Shot_2016-11-16_at_12.28.21_PM.png)  

#### `@media (min-width: $screen-sm)`

![Screen_Shot_2016-11-16_at_12.28.05_PM](/uploads/cf612a36259c248ca597a0850546fe75/Screen_Shot_2016-11-16_at_12.28.05_PM.png)  
![Screen_Shot_2016-11-16_at_12.28.08_PM](/uploads/ae4374f9968c1cc064242fde3ad0e214/Screen_Shot_2016-11-16_at_12.28.08_PM.png)  

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- 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 #18546

See merge request !7429