Fixed showing hidden posts in /comments

If user has no privileges to list the hidden posts, comments on such posts
won't show in /comments anymore.
Fixed binding too many values to PDO statements
More straightforward next/prev post calculation

Instead of getting all three rows at once using abs(id1-id2)<=1, it now asks DB
explicitly about id-1 and id+1. Even though it uses more SQL queries, it's
actually slightly faster.