Accept Mosaico-style unsubscribe URLs (veda-consulting-company#143)

Before
------
As observed in veda-consulting-company#143, the default `versafix-1` templates use tokens like
`[unsubscribe_link]` which do not match `{action.unsubscribeUrl}`. Thus,
when you try to submit the mailing, it fails because the token is missing.

After
-----
The `[unsubscribe_link]` is treated as an alias for
`{action.unsubscribeUrl}`. Thus, when you try to submit the mailing,
it accepts that token.

Comment
-------
There's another required token, `{domain.address}`. For the moment,
the patch waives that requirement because Mosaico UX handles that
differently. See comments inlined.

CRM/Mosaico/MosaicoRequiredTokens.php

@@ -0,0 +1,33 @@
+<?php
+use Civi\FlexMailer\Listener\RequiredTokens;
+
+use CRM_Mosaico_ExtensionUtil as E;
+
+/**
+ * Class CRM_Mosaico_MosaicoRequiredTokens
+ *
+ * The token format for Mosaico extends the traditional format -- all
+ * traditional tokens (eg "{action.unsubscribeUrl}") are supported, and
+ * a few additional aliases (eg "[unsubscribe_link]") are also
+ * supported.
+ *
+ * When validating required tokens, we should accept the aliases.
+ */
+class CRM_Mosaico_MosaicoRequiredTokens extends RequiredTokens {
+
+  public function __construct() {
+    parent::__construct(array('mosaico'), array());
+  }
+
+  public function getRequiredTokens() {
+    return Civi::service('civi_flexmailer_required_tokens')
+      ->getRequiredTokens();
+  }
+
+  public function findMissingTokens($str) {
+    $content = array('body_unspecified' => $str);
+    _mosaico_civicrm_alterMailContent($content);
+    return parent::findMissingTokens($content['body_unspecified']);
+  }
+
+}

CRM/Mosaico/Services.php

@@ -22,6 +22,7 @@ public static function registerServices(ContainerBuilder $container) {
     }
     $container->setDefinition('mosaico_flexmail_composer', new Definition('CRM_Mosaico_MosaicoComposer'));
     $container->setDefinition('mosaico_flexmail_url_filter', new Definition('CRM_Mosaico_UrlFilter'));
+    $container->setDefinition('mosaico_required_tokens', new Definition('CRM_Mosaico_MosaicoRequiredTokens'));
 
     foreach (self::getListenerSpecs() as $listenerSpec) {
       $container->findDefinition('dispatcher')->addMethodCall('addListenerService', $listenerSpec);
@@ -31,6 +32,7 @@ public static function registerServices(ContainerBuilder $container) {
   protected static function getListenerSpecs() {
     $listenerSpecs = array();
 
+    $listenerSpecs[] = array(\Civi\FlexMailer\Validator::EVENT_CHECK_SENDABLE, array('mosaico_required_tokens', 'onCheckSendable'), FM::WEIGHT_MAIN);
     $listenerSpecs[] = array(FM::EVENT_COMPOSE, array('mosaico_flexmail_composer', 'onCompose'), FM::WEIGHT_MAIN);
     $listenerSpecs[] = array(FM::EVENT_COMPOSE, array('mosaico_flexmail_url_filter', 'onCompose'), FM::WEIGHT_ALTER - 100);