Countdown: add a capability to decide who can Create

Summary:
This change adds the `countdown.create` capability to
allow to limit countdown creation.

This change comes from the Wikimedia Phabricator and
was originally proposed by v.bozzolan and implemented
by the kind user 20after4.

https://phabricator.wikimedia.org/T258599

Closes T15208
Ref T15081

Test Plan:
- go to the page /applications/view/PhabricatorCountdownApplication/
- play a bit with the "Can Create Countdowns" option
- note that it is respected

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Tags: #countdown, #policy_archived

Maniphest Tasks: T15208, T15081

Differential Revision: https://we.phorge.it/D25101

src/applications/countdown/application/PhabricatorCountdownApplication.php

@@ -50,6 +50,10 @@ final class PhabricatorCountdownApplication extends PhabricatorApplication {
 
   protected function getCustomCapabilities() {
     return array(
+      PhabricatorCountdownCreateCapability::CAPABILITY => array(
+        'default' => PhabricatorPolicies::POLICY_USER,
+        'caption' => pht('Default create policy for countdowns.'),
+      ),
       PhabricatorCountdownDefaultViewCapability::CAPABILITY => array(
         'caption' => pht('Default view policy for new countdowns.'),
         'template' => PhabricatorCountdownCountdownPHIDType::TYPECONST,

src/applications/countdown/capability/PhabricatorCountdownCreateCapability.php

@@ -0,0 +1,16 @@
+<?php
+
+final class PhabricatorCountdownCreateCapability
+  extends PhabricatorPolicyCapability {
+
+  const CAPABILITY = 'countdown.create';
+
+  public function getCapabilityName() {
+    return pht('Can Create Countdowns');
+  }
+
+  public function describeCapabilityRejection() {
+    return pht('You do not have permission to create a countdown.');
+  }
+
+}

src/applications/countdown/editor/PhabricatorCountdownEditEngine.php

@@ -70,6 +70,11 @@ final class PhabricatorCountdownEditEngine
     return $object->getURI();
   }
 
+  protected function getCreateNewObjectPolicy() {
+    return $this->getApplication()->getPolicy(
+      PhabricatorCountdownCreateCapability::CAPABILITY);
+  }
+
   protected function buildCustomEditFields($object) {
     $epoch_value = $object->getEpoch();
     if ($epoch_value === null) {