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/signup_inner.tmpl

@@ -49,12 +49,10 @@
 					</button>
 				</div>
 			{{end}}
-			{{/* "oauth_container" contains not only "oauth2" methods, but also "OIDC" and "SSPI" methods */}}
-			{{/* TODO: it seems that "EnableSSPI" is only set in "sign-in" handlers, but it should use the same logic to control its display */}}
-			{{$showOAuth2Methods := or .OAuth2Providers .EnableOpenIDSignIn .EnableSSPI}}
-			{{if $showOAuth2Methods}}
+			{{$showExternalAuthMethods := or .OAuth2Providers .EnableOpenIDSignIn .EnableSSPI}}
+			{{if $showExternalAuthMethods}}
 				<div class="divider divider-text">{{ctx.Locale.Tr "sign_in_or"}}</div>
-				{{template "user/auth/oauth_container" .}}
+				{{template "user/auth/external_auth_methods" .}}
 			{{end}}
 		</form>
 	</div>