Add missing filters to remaining link(url) functions in feed.php

[MarcusPope]

All of the url-getting functions in wp-includes/feed.php use rss-specific filters on the resulting url except for the comment_link() & comments_link_feed() functions.

This patch adds a filter for massaging the url before it's sent through esc_url.

This fixes a current bug with the core dev team's recommended solution for root-relative urls, whereby adjusting the filters on get_permalink will inadvertantly and negatively affect rss feeds.

[MarcusPope]

Added filters to comments_link and comment_link functions in feed.php

[nacin]

In 23624:

Add filters to comments_link_feed(), comment_link(), and get_comments_link(). esc_url() the echoed output of comments_link().

props MarcusPope.
fixes #19210.