[Ticket/12555] Make use of canonical urls to avoid duplicate content

Add canonical URL for viewtopic:
* viewtopic.php?t=topic_id
* viewtopic.php?f=forum_id&t=topic_id
* viewtopic.php?f=forum_id&t=topic_id&hilit=highlight_string
* viewtopic.php?f=forum_id&t=topic_id&view=unread
* viewtopic.php?f=forum_id&p=post_id
* viewtopic.php?f=forum_id&t=topic_id&start=0

Add canonical URL for viewforum:
* viewforum.php?f=forum_id
* viewforum.php?f=forum_id&start=0

Add canonical URl for memberlist?mode=viewprofile:
* memberlist.php?mode=viewprofile&u=user_id
* memberlist.php?mode=viewprofile&un=username

https://tracker.phpbb.com/browse/PHPBB3-12555