Redirect to the only OAuth2 provider when no other login methods and fix various problems (#36901)

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)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/user/auth/external_auth_methods.tmpl

@@ -0,0 +1,18 @@
+<div id="external-login-navigator" class="tw-py-1 tw-flex tw-flex-col tw-gap-3">
+	{{range $provider := .OAuth2Providers}}
+		{{/* use QueryEscape for consistent with frontend urlQueryEscape, it is right for a path component */}}
+		<a class="ui button external-login-link tw-gap-3" data-require-appurl-check="true" rel="nofollow" href="{{AppSubUrl}}/user/oauth2/{{$provider.DisplayName | QueryEscape}}">
+			{{$provider.IconHTML 24}} {{ctx.Locale.Tr "sign_in_with_provider" $provider.DisplayName}}
+		</a>
+	{{end}}
+	{{if .EnableOpenIDSignIn}}
+		<a class="ui button external-login-link tw-gap-3" data-require-appurl-check="true" rel="nofollow" href="{{AppSubUrl}}/user/login/openid">
+			{{svg "fontawesome-openid" 24}} {{ctx.Locale.Tr "sign_in_with_provider" "OpenID"}}
+		</a>
+	{{end}}
+	{{if .EnableSSPI}}
+		<a class="ui button external-login-link tw-gap-3" rel="nofollow" href="{{AppSubUrl}}/user/login?auth_with_sspi=1">
+			{{svg "fontawesome-windows" 24}} Windows SSPI
+		</a>
+	{{end}}
+</div>