21778 Correct css styles for pinned navigation

## What does this MR do?
Fixes css for header and comment notes when sidebar is pinned

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-11-08_at_11.02.55_AM](/uploads/ae7bb1f477b2ffbd4d4e0364f2770816/Screen_Shot_2016-11-08_at_11.02.55_AM.png)
After:
![Screen_Shot_2016-11-08_at_11.02.34_AM](/uploads/cf0b9f9f175aa5c6c7e79640b76b1040/Screen_Shot_2016-11-08_at_11.02.34_AM.png)

Before:
![Screen_Shot_2016-11-08_at_11.03.21_AM](/uploads/f48ea742ee6dbb86874e47dd27eddadd/Screen_Shot_2016-11-08_at_11.03.21_AM.png)
After:
![Screen_Shot_2016-11-08_at_11.02.19_AM](/uploads/312b27ba07e04ceb96f8ed9d6f167bee/Screen_Shot_2016-11-08_at_11.02.19_AM.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 #21778 

cc @mikegreiling

See merge request !7362