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