Create simplified user_select.vue shared component

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Why are we doing this work

There are several user_select.vue solutions in the code base today

https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_shared/components/user_select/user_select.vue is already a shared component but it is too complicated for what is needed in most cases and is not flexible enough because it was designed specifically for the sidebar

https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/pages/import/fogbugz/new_user_map/components/user_select.vue and https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/pages/import/fogbugz/new_user_map/components/user_select.vue only work for the project level where one may want to use it for the group/workspace/instance levels. I think these two use cases could be condensed into a single user_select.vue and I also think they could be generalized to work for different namespace levels, but we would need to be able to customize the graphql query and the listbox options. We could maybe name it vue_shared/components/user_select_simple/user_select_simple.vue. But I don't think that needs to be done now.

Another user_select.vue is being created as part of Add individual dropdown (!102838 - merged)

Relevant links

Implementation plan

Verification steps

Edited by 🤖 GitLab Bot 🤖