use Gitlab::UserSettings directly as a singleton instead of including/extending it

Mario de la Ossarequested to merge
ee-37698-current-settings into master

What does this MR do?

Counterpart for gitlab-org/gitlab-ce!16843

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

There were quite a few extra changes required between CE and EE. The main point would be to ensure all usages were accounted for

Edited by Coung Ngo

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