[ticket/13713] Introduce notifications for mentions

PHPBB3-13713

phpBB/language/en/email/mention.txt

@@ -0,0 +1,20 @@
+Subject: Topic reply notification - "{TOPIC_TITLE}"
+
+Hello {USERNAME},
+
+You are receiving this notification because "{AUTHOR_NAME}" mentioned you in the topic "{TOPIC_TITLE}" at "{SITENAME}". You can use the following link to view the reply made.
+
+If you want to view the post where you have been mentioned, click the following link:
+{U_VIEW_POST}
+
+If you want to view the topic, click the following link:
+{U_TOPIC}
+
+If you want to view the forum, click the following link:
+{U_FORUM}
+
+If you no longer wish to receive updates about replies mentioning you, please update your notification settings here:
+
+{U_NOTIFICATION_SETTINGS}
+
+{EMAIL_SIG}