Introduce [user] bb code

As a user of forum I would like to have an ability to write the name of any other user of forum in a bb code so that I quickly mention him and the guy could receive a notification.
If the name of user is found in system, after posting mentioned user should receive notification(if notifications are enabled in mentioned user's profile).

Acceptance Criteria:

• [user]username[/user] should become a link (default style: bold and underlined) to the user's profile
• This feature should work from topics, post. Also it should work in private messages but without sending notifications to mentioned user.
• Mentioned user should receive email notification. He should receive message even in the case he has no view/post/edit permission for topic/post/etc.
• Any user of forum should have an ability to disable email notifications via setting in Profile
• Username inside of [user] is case sensitive
• If I quote someone and there is a text that mentions someone already, then this does become a link, but notifications shouldn't be sent again to mentioned users.
• If there is no such nickname, then that username does not become a link - just a plain text ([user] should not be shown anyway).
• If we edit some text, then mentioned people don't get notification again.
• On preview page we see links already

Notification templates:
EN: You were mentioned in topic "here we must put link to post".
RU: Вы были упомянуты в следущей теме "здесь нужно указать ссылку на пост".
UK: Вас згадали в темі: "тут потрібно вказати посилання на тему"

Note that sending notifications also depends on configuration from poulpe.