Themes: Allow passing a custom class to post and comment pagination functions.

This affects:
* `get_the_post_navigation()`
* `get_the_posts_navigation()`
* `get_the_posts_pagination()`
* `get_the_comments_navigation()`
* `get_the_comments_pagination()`

Props jfarthing84, markparnell, davidbaumwald, SergeyBiryukov.
Fixes #39378.
Built from https://develop.svn.wordpress.org/trunk@48378


git-svn-id: https://core.svn.wordpress.org/trunk@48147 1a063a9b-81f0-0310-95a4-ce76da25c4cd