Replaced duplicate z-index with index of 400

Merged Luke "Jared" Bennett requested to merge fix-header-z-index into master

What does this MR do?

Adds z-index 400 to header to stop the user dropdown showing underneath the sidebar.

This issue isn't actually present on CE, but this is the backport from EE.

CE port of: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1938

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

Why was this MR needed?

Screenshots (if relevant)

Screen_Shot_2017-05-19_at_14.02.44

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32609

Edited by Luke "Jared" Bennett