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/signin_inner.tmpl

@@ -46,14 +46,13 @@
 				</button>
 			</div>
 		</form>
-		{{end}}{{/*if .EnablePasswordSignInForm*/}}
-		{{/* "oauth_container" contains not only "oauth2" methods, but also "OIDC" and "SSPI" methods */}}
-		{{$showOAuth2Methods := or .OAuth2Providers .EnableOpenIDSignIn .EnableSSPI}}
-		{{if and $showOAuth2Methods .EnablePasswordSignInForm}}
+		{{end}}{{/*end if .EnablePasswordSignInForm*/}}
+		{{$showExternalAuthMethods := or .OAuth2Providers .EnableOpenIDSignIn .EnableSSPI}}
+		{{if and $showExternalAuthMethods .EnablePasswordSignInForm}}
 			<div class="divider divider-text">{{ctx.Locale.Tr "sign_in_or"}}</div>
 		{{end}}
-		{{if $showOAuth2Methods}}
-			{{template "user/auth/oauth_container" .}}
+		{{if $showExternalAuthMethods}}
+			{{template "user/auth/external_auth_methods" .}}
 		{{end}}
 	</div>
 </div>