Fixes: #36846
1. When there is only on OAuth2 login method, automatically direct to it
2. Fix legacy problems in code, including:
* Rename template filename and fix TODO comments
* Fix legacy variable names
* Add missing SSPI variable for template
* Fix unnecessary layout, remove garbage styles
* Only do AppUrl(ROOT_URL) check when it is needed (avoid unnecessary
warnings to end users)
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
