Closes https://github.com/go-gitea/gitea/issues/39287
Security key nicknames could only be set at registration, so a skipped
nickname left an auto-generated hex name until the key was
re-registered. Each key now has a Rename button opening a dialog with
the current nickname. A nickname used by another of the user's keys
(case-insensitive) or a blank nickname is rejected. Renames are recorded
as `user:webauth:rename` audit events.
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>