[ticket/12396] Add Template events viewforum_forum_name_append/prepend

Since we have `<!-- EVENT viewtopic_topic_title_prepend -->` (although not the corresponding append yet), it might be useful to have the forum name as an event as well. It might be nice to be able to add the forum icon (as set in the ACP) before it (or some other stuff in the future).

p.s. Also added the missing viewtopic_topic_title_append

PHPBB3-12396