Set navbar-gitlab z-index to 400
What does this MR do?
CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11547
Fixes issue where user dropdown is under the sidebar.
Are there points in the code the reviewer needs to double check?
Why was this MR needed?
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Documentation created/updated -
API support added - Tests
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Added for this feature/bug -
All builds are passing
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Conform by the merge request performance guides -
Conform by the style guides -
Branch has no merge conflicts with master(if it does - rebase it please) -
Squashed related commits together