Show a notice to admin if network activation

Mailpoet seems to be working on network activated multisites but we
don't support it.
[MAILPOET-923]

lib/Config/DeferredAdminNotices.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace MailPoet\Config;
+
+use MailPoet\WP\Notice;
+
+class DeferredAdminNotices {
+
+  const OPTIONS_KEY_NAME = 'mailpoet_deferred_admin_notices';
+
+  /**
+   * @param string $message
+   */
+  public function addNetworkAdminNotice($message) {
+    $notices = get_option(DeferredAdminNotices::OPTIONS_KEY_NAME, array());
+    $notices[] = array(
+      "message" => $message,
+      "networkAdmin" => true,// if we'll need to display the notice to anyone else
+    );
+    update_option(DeferredAdminNotices::OPTIONS_KEY_NAME, $notices);
+  }
+
+  public function flushAll() {
+    $notices = get_option(DeferredAdminNotices::OPTIONS_KEY_NAME, array());
+
+    foreach($notices as $notice) {
+      $notice = new Notice(Notice::TYPE_WARNING, $notice["message"]);
+      add_action('network_admin_notices', array($notice, 'displayWPNotice'));
+    }
+
+    if(!empty($notices)) {
+      delete_option(DeferredAdminNotices::OPTIONS_KEY_NAME);
+    }
+  }
+
+}