Presently, attempting to rename a non-local (e.g. Oauth2 or LDAP) user
results in an error, even if the requester is an administrator. As far
as I can tell, this is a security feature, not architectural in nature,
as automatic account linking could be used to take control of another
user's account. This is not a concern for an administrator, who we
should trust to know what they are doing.
This patch allows admins, and only admins, to rename non-local users.
Fixes https://github.com/go-gitea/gitea/issues/18308 (sort of)
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>