Adds the settings pages to create OAuth2 apps also to the org settings
and allows to create apps for orgs.
Refactoring: the oauth2 related templates are shared for
instance-wide/org/user, and the backend code uses `OAuth2CommonHandlers`
to share code for instance-wide/org/user.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
