login: Hide email/password fields if email auth is disabled

Fixes zulip#1991.

Use the email_auth_enabled server setting to conditionally render
the username/password fields and the alternative auth divider.

Author: yash-agarwa-l

lib/widgets/login.dart

@@ -446,11 +446,14 @@ class _LoginPageState extends State<LoginPage> {
     final zulipLocalizations = ZulipLocalizations.of(context);
 
     final externalAuthenticationMethods = widget.serverSettings.externalAuthenticationMethods;
+    final emailAuthEnabled = widget.serverSettings.emailAuthEnabled;
 
     final loginContent = Column(mainAxisAlignment: MainAxisAlignment.center, children: [
-      _UsernamePasswordForm(loginPageState: this),
+      if (emailAuthEnabled)
+        _UsernamePasswordForm(loginPageState: this),
       if (externalAuthenticationMethods.isNotEmpty) ...[
-        _AlternativeAuthDivider(),
+        if (emailAuthEnabled)
+          _AlternativeAuthDivider(),
         ...externalAuthenticationMethods.map((method) {
           final icon = method.displayIcon;
           return OutlinedButton.icon(

test/widgets/login_test.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:flutter_checks/flutter_checks.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:http/http.dart' as http;
 import 'package:zulip/api/core.dart';
@@ -356,6 +357,56 @@ void main() {
       // TODO test _inProgress logic
     });
 
+    group('email auth disabled', () {
+      testWidgets('hides username/password fields and login button', (tester) async {
+        final serverSettings = eg.serverSettings(emailAuthEnabled: false);
+        await prepare(tester, serverSettings);
+        check(findUsernameInput).findsNothing();
+        check(findPasswordInput).findsNothing();
+        check(findSubmitButton).findsNothing();
+      });
+
+      testWidgets('shows external auth methods without divider', (tester) async {
+        final method = ExternalAuthenticationMethod(
+          name: 'google',
+          displayName: 'Google',
+          displayIcon: null,
+          loginUrl: '/accounts/login/social/google',
+          signupUrl: '/accounts/register/social/google',
+        );
+        final serverSettings = eg.serverSettings(
+          emailAuthEnabled: false,
+          externalAuthenticationMethods: [method]);
+        await prepare(tester, serverSettings);
+        check(findUsernameInput).findsNothing();
+        check(findPasswordInput).findsNothing();
+        check(findSubmitButton).findsNothing();
+        check(find.textContaining('Google')).findsOne();
+        final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+        check(find.text(zulipLocalizations.loginMethodDivider)).findsNothing();
+      });
+
+      testWidgets('shows divider when email auth enabled with external methods', (tester) async {
+        final method = ExternalAuthenticationMethod(
+          name: 'google',
+          displayName: 'Google',
+          displayIcon: null,
+          loginUrl: '/accounts/login/social/google',
+          signupUrl: '/accounts/register/social/google',
+        );
+        final serverSettings = eg.serverSettings(
+          emailAuthEnabled: true,
+          externalAuthenticationMethods: [method]);
+        await prepare(tester, serverSettings);
+        check(findUsernameInput).findsOne();
+        check(findPasswordInput).findsOne();
+        check(findSubmitButton).findsOne();
+        check(find.textContaining('Google')).findsOne();
+        final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+        check(find.text(zulipLocalizations.loginMethodDivider)).findsOne();
+      });
+    });
+
     group('web auth', () {
       testWidgets('basic happy case', (tester) async {
         final method = ExternalAuthenticationMethod(