fix: retrieve users authorities in user management page (reactive)

[chrisdns]

fix #14475

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[pascalgrimaud]

Is it possible to add an integration test in UserServiceIT plz? We shouldn't have this issue if there was a test for that

Anyway, thanks @chrisdns
cc @mraible @cbornet @murdos for the review of this important fix

[pascalgrimaud]

I'm merging this as it's better than what we currently have, but still missing tests here

[chrisdns]

https://opencollective.com/generator-jhipster/expenses/37187