Check all namespaces on validation of new username.
What does this MR do?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/24519 and https://gitlab.com/gitlab-org/gitlab-ce/issues/24580
Screenshots (if relevant)
(disregard the gray text. It's a gifox thing)
Does this MR meet the acceptance criteria?
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Changelog entry added -
All builds are passing -
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