Made "opencase_cases" and "opencase_no_cases" modules and gave the latter a version of the contextual menu

modules/opencase_cases/opencase_cases.info.yml

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+name: 'Opencase Cases'
+type: module
+description: 'Provides case entities and a required field on activities linking them to a case.'
+core: 8.x
+package: 'OpenCase'
+dependencies:
+  - opencase_entities

modules/opencase_cases/opencase_cases.module

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+<?php
+
+/**
+ * @file
+ * Contains opencase_cases.module.
+ */
+
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Implements hook_help().
+ */
+function opencase_cases_help($route_name, RouteMatchInterface $route_match) {
+  switch ($route_name) {
+    // Main module help for the opencase_cases module.
+    case 'help.page.opencase_cases':
+      $output = '';
+      $output .= '<h3>' . t('About') . '</h3>';
+      $output .= '<p>' . t('Provides case entities and a required field on activities linking them to a case.') . '</p>';
+      return $output;
+
+    default:
+  }
+}
+
+/**
+ * Implements hook_theme().
+ */
+function opencase_cases_theme() {
+  return [
+    'opencase_cases' => [
+      'render element' => 'children',
+    ],
+  ];
+}
+
+function opencase_cases_entity_base_field_info($entity_type) {
+  $fields = array();
+  
+  // Add Case field to Activity 
+  if ($entity_type->id() === 'oc_activity') {
+    $fields['oc_case'] = BaseFieldDefinition::create('entity_reference')
+      ->setLabel(t('Case'))
+      ->setDescription(t('The case this activity belongs to.'))
+      ->setSetting('target_type', 'oc_case')
+      ->setSetting('handler', 'default')
+      ->setTranslatable(TRUE)
+      ->setCardinality(1)
+      ->setDefaultValueCallback('opencase_cases_default_activity_case_value')
+      ->setDisplayOptions('view', [
+        'type' => 'string',
+        'weight' => -3,
+      ])
+      ->setRequired(TRUE);
+  }
+  return $fields;
+}
+
+/**
+ * When creating an activity, it sets the case id from the URL.
+ */
+public static function opencase_cases_default_activity_case_value() {
+  return array(\Drupal::request()->query->get('case_id'));
+}

modules/opencase_cases/templates/opencase-cases.html.twig

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+<!-- Add you custom twig html here -->