Issue #2199875 by andypost: No link to add view|form mode for entity types...

Issue #2199875 by andypost: No link to add view|form mode for entity types that have no shipped modes.

core/modules/entity/entity.local_actions.yml

+entity.view_mode_add:
+  route_name: entity.view_mode_add
+  title: 'Add new view mode'
+  weight: 1
+  appears_on:
+    - entity.view_mode_list
+
+entity.form_mode_add:
+  route_name: entity.form_mode_add
+  title: 'Add new form mode'
+  weight: 1
+  appears_on:
+    - entity.form_mode_list

core/modules/entity/lib/Drupal/entity/Tests/EntityDisplayModeTest.php

@@ -39,6 +39,8 @@ public function testEntityViewModeUI() {
     $this->drupalLogin($this->drupalCreateUser(array('administer display modes')));
     $this->drupalGet('admin/structure/display-modes/view');
     $this->assertResponse(200);
+    $this->assertText(t('Add new view mode'));
+    $this->assertLinkByHref('admin/structure/display-modes/view/add');
 
     $this->drupalGet('admin/structure/display-modes/view/add/entity_test_mulrev');
     $this->assertResponse(404);
@@ -75,6 +77,8 @@ public function testEntityFormModeUI() {
     $this->drupalLogin($this->drupalCreateUser(array('administer display modes')));
     $this->drupalGet('admin/structure/display-modes/form');
     $this->assertResponse(200);
+    $this->assertText(t('Add new form mode'));
+    $this->assertLinkByHref('admin/structure/display-modes/form/add');
 
     $this->drupalGet('admin/structure/display-modes/form/add/entity_test_no_label');
     $this->assertResponse(404);