Restrict header logo to 36px so it doesn't overflow.

## What does this MR do?

Fixes the logo in the header.

## Are there points in the code the reviewer needs to double check?

I don't think so.

## Why was this MR needed?

Logos that weren't 36px by 36px were broken in 8.9.0

## What are the relevant issue numbers?

Fixes #18980

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-06-22_at_1.52.54_PM](/uploads/de6e4d9ccf40b972565e3864173a9907/Screen_Shot_2016-06-22_at_1.52.54_PM.png)

After:

![Screen_Shot_2016-06-22_at_1.52.37_PM](/uploads/d72e015263888b5155c1db5e25f54d66/Screen_Shot_2016-06-22_at_1.52.37_PM.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc: @jschatz1 @dzaporozhets @rspeicher

See merge request !4861