Handled fetching staff users for email alerts

refs TryGhost/Team#1826

- adds a method on user model which fetches all eligible users for a type of email alert
- restricts users to active `Owner` and `Administrators` with setting turned on

ghost/core/core/server/models/user.js

@@ -488,6 +488,32 @@ User = ghostBookshelf.Model.extend({
         return query.fetch(options);
     },
 
+    /**
+     * Returns users who should receive a specific type of alert
+     * @param {'free-signup'|'paid-started'|'paid-canceled'} type The type of alert to fetch users for
+     * @param {any} options
+     * @return {Promise<[Object]>} Array of users
+     */
+    getEmailAlertUsers(type, options) {
+        options = options || {};
+
+        let filter = 'status:active';
+        if (type === 'free-signup') {
+            filter += '+free_member_signup_notification:true';
+        } else if (type === 'paid-started') {
+            filter += '+paid_subscription_started_notification:true';
+        } else if (type === 'paid-canceled') {
+            filter += '+paid_subscription_canceled_notification:true';
+        }
+        return this.findAll(_.merge({filter, withRelated: ['roles']}, options)).then((users) => {
+            return users.toJSON().filter((user) => {
+                return user?.roles?.some((role) => {
+                    return ['Owner', 'Administrator'].includes(role.name);
+                });
+            });
+        });
+    },
+
     /**
      * ### Edit
      *

ghost/core/test/unit/server/models/user.test.js

@@ -534,6 +534,34 @@ describe('Unit: models/user', function () {
         });
     });
 
+    describe('getEmailAlertUsers', function () {
+        beforeEach(function () {
+            sinon.stub(models.User, 'findAll');
+        });
+
+        it('can filter out only Admin and Owner users', function () {
+            const users = sinon.stub();
+
+            users.toJSON = sinon.stub().returns([
+                testUtils.permissions.owner.user,
+                testUtils.permissions.admin.user,
+                testUtils.permissions.editor.user,
+                testUtils.permissions.author.user,
+                testUtils.permissions.contributor.user
+            ]);
+
+            models.User
+                .findAll
+                .resolves(users);
+
+            return models.User.getEmailAlertUsers('free-signup', {}).then((alertUsers) => {
+                alertUsers.length.should.eql(2);
+                alertUsers[0].roles[0].name.should.eql('Owner');
+                alertUsers[1].roles[0].name.should.eql('Administrator');
+            });
+        });
+    });
+
     describe('isSetup', function () {
         it('active', function () {
             sinon.stub(models.User, 'getOwnerUser').resolves({get: sinon.stub().returns('active')});