Menu editor does not load

The menu editor does not load anymore because '_' cannot be found.

Steps to reproduce:
1. login to the archetype CMS
2. edit the only channel
3. edit the main menu via the 'Edit Menu' button

Expected: the menu editor is shown
Actual: a spinner is shown, and the following errors are logged in the JavaScript console:

Failed to load resource: the server responded with a status of 404 (Not Found) http://localhost:8080/cms/angular/components/underscore/underscore-min.js
ReferenceError: _ is not defined
at addCollapsedProperties (http://localhost:8080/cms/angular/menu/services/MenuService.c8fa310a.js:43:21)
at http://localhost:8080/cms/angular/menu/services/MenuService.c8fa310a.js:58:37
at http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:79:290
at http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:109:171
at h.$eval (http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:123:67)
at h.$digest (http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:120:140)
at h.$apply (http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:123:334)
at n (http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:80:80)
at O (http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:84:150)
at XMLHttpRequest.D.onload (http://localhost:8080/cms/angular/components/angular/angular.min.4461ff2a.js:85:169) angular.js:11339