Fixed flash of button error state when redirected to 2fa screen

closes https://linear.app/tryghost/issue/ENG-1652/

- returning `undefined` from a task is equivalent to failing
- switched to returning `true` when we get the 2fa required error so the button stays in the neutral/success state
- added `SUCCESS` and `FAILURE` consts to better reflect control flow when returning from tasks and ensured we always return a value

ghost/admin/app/controllers/signin.js

@@ -1,6 +1,3 @@
-// TODO: bump lint rules to be able to take advantage of https://github.com/ember-cli/eslint-plugin-ember/issues/560
-/* eslint-disable ghost/ember/alias-model-in-controller */
-
 import Controller, {inject as controller} from '@ember/controller';
 import ValidationEngine from 'ghost-admin/mixins/validation-engine';
 import {action} from '@ember/object';
@@ -13,6 +10,10 @@ import {inject as service} from '@ember/service';
 import {task} from 'ember-concurrency';
 import {tracked} from '@glimmer/tracking';
 
+const SUCCESS = true;
+const FAILURE = false;
+
+/* eslint-disable ghost/ember/alias-model-in-controller */
 export default class SigninController extends Controller.extend(ValidationEngine) {
     @controller application;
 
@@ -53,17 +54,18 @@ export default class SigninController extends Controller.extend(ValidationEngine
     @task({drop: true})
     *authenticateTask(authStrategy, {identification, password}) {
         try {
-            return yield this.session
-                .authenticate(authStrategy, {identification, password})
-                .then(() => true); // ensure task button transitions to "success" state
+            yield this.session.authenticate(authStrategy, {identification, password});
+            return SUCCESS;
         } catch (error) {
             if (isForbiddenError(error)) {
+                // login was successful, but 2FA verification is required
                 this.router.transitionTo('signin-verify');
-                return;
+                return true;
             }
 
             if (isVersionMismatchError(error)) {
-                return this.notifications.showAPIError(error);
+                this.notifications.showAPIError(error);
+                return FAILURE;
             }
 
             this.signin.errors.clear();
@@ -101,7 +103,7 @@ export default class SigninController extends Controller.extend(ValidationEngine
                 );
             }
 
-            return false;
+            return FAILURE;
         }
     }
 
@@ -120,6 +122,7 @@ export default class SigninController extends Controller.extend(ValidationEngine
                 .perform('authenticator:cookie', {identification, password});
         } catch (error) {
             this.flowErrors = 'Please fill out the form to sign in.';
+            return FAILURE;
         }
     }
 
@@ -138,15 +141,17 @@ export default class SigninController extends Controller.extend(ValidationEngine
             yield this.validate({property: 'forgotPassword'});
             yield this.ajax.post(forgottenUrl, {data: {password_reset: [{email}]}});
             this.flowNotification = 'An email with password reset instructions has been sent.';
-            return true;
+            return SUCCESS;
         } catch (error) {
             // ValidationEngine throws "undefined" for failed validation
             if (!error) {
-                return this.flowErrors = 'We need your email address to reset your password.';
+                this.flowErrors = 'We need your email address to reset your password.';
+                return FAILURE;
             }
 
             if (isVersionMismatchError(error)) {
-                return notifications.showAPIError(error);
+                notifications.showAPIError(error);
+                return FAILURE;
             }
 
             if (error && error.payload && error.payload.errors && isEmberArray(error.payload.errors)) {
@@ -160,6 +165,8 @@ export default class SigninController extends Controller.extend(ValidationEngine
             } else {
                 notifications.showAPIError(error, {defaultErrorText: 'There was a problem with the reset, please try again.', key: 'forgot-password.send'});
             }
+
+            return FAILURE;
         }
     }
 }