Site admins cannot add access tokens for any user by default
Created by: ryanslade
By default, when any user can create access tokens, do not allow site admins to create tokens for other users. This would allow them to potentially use the access token to gain access to the user's private code.
Update usages of CheckSiteAdminOrSameUser and switch to CheckSameUser where it might expose private code.
Part of https://github.com/sourcegraph/sourcegraph/issues/20983