[ticket/12555] Allow to set canonical URLs to avoid duplicate content

PHPBB3-12555
[ticket/12555] Add canonical URl for memberlist mode=viewprofile

Avoids duplicate content of:
* memberlist.php?mode=viewprofile&u=user_id
* memberlist.php?mode=viewprofile&un=username

PHPBB3-12555
[ticket/12555] Add canonical URL for viewforum

Avoids duplicate content of:
* viewforum.php?f=forum_id
* viewforum.php?f=forum_id&start=0

PHPBB3-12555
[ticket/12555] Add canonical URL for viewtopic

Avoids duplicate content of:
* 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

PHPBB3-12555
Merge pull request #2468 from nickvergessen/ticket/12555

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

* nickvergessen/ticket/12555:
  [ticket/12555] Add canonical URL for viewtopic
  [ticket/12555] Add canonical URL for viewforum
  [ticket/12555] Add canonical URl for memberlist mode=viewprofile
  [ticket/12555] Allow to set canonical URLs to avoid duplicate content