Make sure to use the user name field everywhere
Add showing and modifying user name
Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/306
Co-Authored-By: konrad <konrad@kola-entertainments.de>
Co-Committed-By: konrad <konrad@kola-entertainments.de>