Fixed positioning of hamburger menu on header

Hey guys just noticed the hamburger menu for the top navigation was mis-aligned on 
`gitlab-ce:master` and `gitlab.com`.
I was not able to find an open issue or merge request directly referencing this issue so thought I would add a quick fix.

### Changes

This MR changes the vertical positioning from `top` to vertical margins (Like in bootstrap itself).
 - Also added a minor fix to remove `border-top` from the collapse menu so its the same height as the navigation.
 - finally added an active class to the hamburger toggle so you can see when its open.

This changes an element in `app/views/layouts/header/_default.html.haml` and thus would impact most pages behind login.

## Screenshots

### Before

![before](https://gitlab.com/hanloong/gitlab-ce/uploads/e3d4631dd0420ec0e9048c435a2f734b/before.png)

### After

![after](https://gitlab.com/hanloong/gitlab-ce/uploads/7c93c3ef1b01fbefc39c5a1ed61c2d1b/after.png)

Happy to discuss any changes/suggestions.

Thanks

See merge request !1541