Flash notifications should not be hidden behind other elements

Summary

Sticky flash notifications were introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30141. These look great but they are hidden behind other elements like the merge request tabs.

Steps to reproduce

Try triggering an error or information flash notification in a merge request. You could try clicking Unsubscribe from one of the emails from a merge request.

Relevant logs and/or screenshots

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