Comment pagination settings should discourage infinite comments-per-page

[boonebgorges]

The default value for page_comments is 0. See #12562. When a comment has hundreds or thousands or trillions of comments, the lack of pagination causes severe performance problems. Part of the spiritual journey described in #8071 involved forcing pagination. We tried a few things in #8071, but decided to leave the problem for a separate ticket.

A couple of possible ways forward, some of which are mutually compatible:

• Force comment pagination in all cases. This could be coupled with a bump in the default value of comments_per_page. Good: It's consistent, and it allows us to drop a checkbox from the UI. Bad: It breaks comment permalinks for many posts (though see #34106).
• Force comment pagination only when the number of comments on a post is dangerously high. Good: It reduces permalink breakage. Bad: It's confusing for the user.
• Do nothing for existing installations, but enable page_comments by default on new installs. This could be coupled with a bump in the default value of comments_per_page.
• Add some sort of AYS message to options-discussion.php that discourages admins from disabling page_comments, or setting comments_per_page too high.