Comments have three states: visible, hidden, and invisible. Visible
comments are always shown. Hidden comments are not shown until the user
clicks 'Show all comments'. Invisible comments are never shown to the
user. Deleted comments are treated as hidden for moderators and
invisible for normal users. Thresholded comments are treated as hidden
for all users.
Certain parts of comment rendering triggered sql queries that we didn't
really need to do. Rework things to avoid this.
* Preload comment creators in order to display commenter names with link_to_user.
* Preload comment votes in order to display "undo vote" links. Only preload
votes for members since anonymous users can't vote and don't have "undo
vote" links.
* Rework various conditionals to do the filtering in Ruby so that we
avoid issuing any extra queries in sql.
* Avoid issuing any queries at all when the post doesn't have any
comments (when last_commented_at is blank).
