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

After

Happy to discuss any changes/suggestions.

Thanks