#11124 Fix/employee selector dropdown overlay

Fixes the employee dropdown in the header looking faded and hard to read while the dashboard is still loading.
The loading effect on the dashboard cards was showing on top of the open list; it now stays behind it.
This also keeps other dropdowns and the top header from being covered while a page loads.

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Summary by cubic

Fixes the header employee selector dropdown looking faded while dashboard cards are still loading. The Nebular spinner veil now layers below dropdown overlays, so the header and open dropdowns stay readable during page loads.