Show user display name instead of username everywhere

This is postponed on 🐛 Fix test pipeline issues Active

🐛 Fix test pipeline issues Active is merged

Ready for review. Not that in some places, especially in tests, we're still using ::getAccountName() because that belongs to user selector widget and fixing that is not in the scope of this issue.

While working on this I've opened 📌 Unify verb: ban/unban -> block/unblock Active as a followup.

The user display name is being shown instead of the username everywhere (except user selection autocomplete as mentioned in the issue's description). The code is good with many nice improvements. I mark as RTBC.

Thank you for review