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>

services/auth/source/oauth2/providers_base.go

@@ -42,10 +42,10 @@ func (b *BaseProvider) IconHTML(size int) template.HTML {
 	case "github":
 		svgName = "octicon-mark-github"
 	}
-	svgHTML := svg.RenderHTML(svgName, size, "tw-mr-2")
+	svgHTML := svg.RenderHTML(svgName, size)
 	if svgHTML == "" {
 		log.Error("No SVG icon for oauth2 provider %q", b.name)
-		svgHTML = svg.RenderHTML("gitea-openid", size, "tw-mr-2")
+		svgHTML = svg.RenderHTML("gitea-openid", size)
 	}
 	return svgHTML
 }