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:

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?

What are the relevant issue numbers?


Edited by Luke "Jared" Bennett