Move dynamic segments from Premium plugin

[MAILPOET-2382]

lib/DynamicSegments/Persistence/Loading/SubscribersCount.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace MailPoet\Premium\DynamicSegments\Persistence\Loading;
+
+use MailPoet\Models\Subscriber;
+use MailPoet\Premium\Models\DynamicSegment;
+
+class SubscribersCount {
+
+  /**
+   * @param DynamicSegment $dynamic_segment
+   *
+   * @return int
+   */
+  function getSubscribersCount(DynamicSegment $dynamic_segment) {
+    $orm = Subscriber::selectExpr('count(distinct ' . Subscriber::$_table . '.id) as cnt');
+    foreach ($dynamic_segment->getFilters() as $filter) {
+      $orm = $filter->toSql($orm);
+    }
+    return $orm->findOne()->cnt;
+  }
+
+}