Use correct manual work package form path

The new controller builds its own API resource using the
static `/projects/:identifier/work_packages/form` path,
which was expanded with the `/api/v3` prefix until 0c768829e7.

With this commit, the resource was invalid and causes new work packages to be broken.
In order to restore the manual path, a new path helper was created.

frontend/app/helpers/path-helper.js

@@ -233,12 +233,18 @@ module.exports = function() {
     apiV3WorkPackagePath: function(workPackageId) {
       return PathHelper.apiV3 + '/work_packages/' + workPackageId;
     },
+    apiV3WorkPackageFormPath: function(projectIdentifier) {
+      return PathHelper.apiv3ProjectWorkPackagesPath(projectIdentifier) + '/form';
+    },
     apiPrioritiesPath: function() {
       return PathHelper.apiV3 + '/priorities';
     },
     apiV3ProjectsPath: function(projectIdentifier) {
       return PathHelper.apiV3 + PathHelper.projectsPath() + '/' + projectIdentifier;
     },
+    apiv3ProjectWorkPackagesPath: function(projectIdentifier) {
+      return PathHelper.apiV3ProjectsPath(projectIdentifier) + '/work_packages';
+    },
     apiV3ProjectCategoriesPath: function(projectIdentifier) {
       return PathHelper.apiV3ProjectsPath(projectIdentifier) + '/categories';
     },

frontend/app/services/work-package-service.js

@@ -88,9 +88,9 @@ module.exports = function($http,
         links: {
           update: HALAPIResource
             .setup(PathHelper
-              .projectWorkPackagesFormPath(projectIdentifier)),
+              .apiV3WorkPackageFormPath(projectIdentifier)),
           updateImmediately: HALAPIResource.setup(
-            PathHelper.projectWorkPackagesPath(projectIdentifier),
+            PathHelper.apiv3ProjectWorkPackagesPath(projectIdentifier),
             { method: 'post' }
           )
         }