Comment paging

[ryan]

Page comments when viewing posts, pages, and attachments.

Partial implementation.

Implement previous_comments_link() and add options for paging and threading

Usable implementation with options for setting comments per page and threading depth in Settings->Discussion

(In [8961]) First cut of comment paging. Add paging and threading settings. see #7769 #7635

(In [8964]) Fix comment display when paging is off. see #7769

(In [9097]) paginate_comments_links(). see #7769

Added paginate_comments_links(), which wraps paginate_links().