org settings: Add organization profile preview option.

This should make it convenient and obvious how verify that their
organization profile looks nice after being markdown-rendered.

Fixes #12105.

static/styles/settings.scss

@@ -495,6 +495,7 @@ input[type=checkbox] + .inline-block {
     margin-bottom: 20px;
 }
 
+#id_org_profile_preview,
 .realm-icon-section {
     margin-bottom: 20px;
 }

static/templates/settings/organization-profile-admin.handlebars

@@ -45,6 +45,10 @@
                   id="realm_icon_delete_button">{{t 'Delete profile picture' }}</button>
             </div>
         </div>
+        <a href="/login/?preview=true" target="_blank" class="button rounded sea-green w-200 block" id="id_org_profile_preview">
+            {{t 'Preview organization profile' }}
+            <i class="fa fa-external-link" aria-hidden="true" title="{{t 'Preview organization profile' }}"></i>
+        </a>
 
         <div class="subsection-header">
             <h3>{{t "Organization logo" }}

zerver/tests/test_auth_backends.py

@@ -216,6 +216,31 @@ class AuthBackendTest(ZulipTestCase):
                                                               password,
                                                               realm=get_realm("zulip")))
 
+    def test_login_preview(self) -> None:
+        # Test preview=true displays organization login page
+        # instead of redirecting to app
+        self.login(self.example_email("iago"))
+        realm = get_realm("zulip")
+        result = self.client_get('/login/?preview=true')
+        self.assertEqual(result.status_code, 200)
+        self.assert_in_response(realm.description, result)
+        self.assert_in_response(realm.name, result)
+        self.assert_in_response("Log in to Zulip", result)
+
+        data = dict(description=ujson.dumps("New realm description"),
+                    name=ujson.dumps("New Zulip"))
+        result = self.client_patch('/json/realm', data)
+        self.assert_json_success(result)
+
+        result = self.client_get('/login/?preview=true')
+        self.assertEqual(result.status_code, 200)
+        self.assert_in_response("New realm description", result)
+        self.assert_in_response("New Zulip", result)
+
+        result = self.client_get('/login/')
+        self.assertEqual(result.status_code, 302)
+        self.assertEqual(result.url, 'http://zulip.testserver')
+
     @override_settings(AUTHENTICATION_BACKENDS=('zproject.backends.ZulipDummyBackend',))
     def test_no_backend_enabled(self) -> None:
         result = self.client_get('/login/')

zerver/views/auth.py

@@ -689,10 +689,16 @@ class TwoFactorLoginView(BaseTwoFactorLoginView):
             return super().done(form_list, **kwargs)
 
 def login_page(request: HttpRequest, **kwargs: Any) -> HttpResponse:
+    # To support previewing the Zulip login pages, we have a special option
+    # that disables the default behavior of redirecting logged-in users to the
+    # logged-in app.
+    is_preview = False
+    if request.method == "GET" and request.GET and request.GET.get('preview'):
+        is_preview = True
     if settings.TWO_FACTOR_AUTHENTICATION_ENABLED:
         if request.user and request.user.is_verified():
             return HttpResponseRedirect(request.user.realm.uri)
-    elif request.user.is_authenticated:
+    elif request.user.is_authenticated and not is_preview:
         return HttpResponseRedirect(request.user.realm.uri)
     if is_subdomain_root_or_alias(request) and settings.ROOT_DOMAIN_LANDING_PAGE:
         redirect_url = reverse('zerver.views.registration.realm_redirect')