Add sidebar assignee service

spec/javascripts/vue_sidebar_assignees/services/sidebar_assignees_service_spec.js

+import SidebarAssigneesService from '~/vue_sidebar_assignees/services/sidebar_assignees_service';
+
+describe('SidebarAssigneesService', () => {
+  let service;
+
+  beforeEach(() => {
+    service = new SidebarAssigneesService('', 'field');
+  });
+
+  describe('constructor', () => {
+    it('should save field', () => {
+      expect(service.field).toEqual('field');
+    });
+
+    it('should save sidebarAssigneeResource', () => {
+      expect(service.sidebarAssigneeResource).toBeDefined();
+    });
+  });
+
+  describe('update', () => {
+    it('should call vue resource update', (done) => {
+      const userIds = [1, 2, 3];
+
+      spyOn(service.sidebarAssigneeResource, 'update').and.callFake((o) => {
+        expect(o.field).toEqual(userIds);
+        done();
+      });
+
+      service.update(userIds);
+    });
+  });
+});