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.go

@@ -62,7 +62,7 @@ func (p *AuthSourceProvider) DisplayName() string {
 
 func (p *AuthSourceProvider) IconHTML(size int) template.HTML {
 	if p.iconURL != "" {
-		img := fmt.Sprintf(`<img class="tw-object-contain tw-mr-2" width="%d" height="%d" src="%s" alt="%s">`,
+		img := fmt.Sprintf(`<img class="tw-object-contain" width="%d" height="%d" src="%s" alt="%s">`,
 			size,
 			size,
 			html.EscapeString(p.iconURL), html.EscapeString(p.DisplayName()),