MDL-50109 forum: Respect chronological order of blog posts in prev/next

MDL-50109 forum: Respect chronological order of blog posts in prev/next

The function forum_get_discussion_neighbours() is used to generate links
to neighbour discussions. I had to change the signature of the function
to have access to the forum's type without the need to re-read it from
the database in additional query.

The SQL query for blogs is a variant of the existing query, just using
the first post's creation time instead of the discussion's timemodified
time.