Tech debt: migrate UserTabs to Vue

Problem to solve

app/assets/javascripts/pages/users/user_tabs.js has a lot of jQuery and legacy JavaScript. It is also quite hard to follow. We should migrate this to Vue to reduce technical debt.

This should be completed after #23466 (closed) since app/assets/javascripts/pages/groups/shared/group_tabs.js relies on app/assets/javascripts/pages/users/user_tabs.js.